,324 DR. MARTIN BARRY'S RESEARCHES IN ExMBRYOLOGY. 



The Ovum in the centre of the Ovisac. Manner of origin of the Memhrana granulosa. 



75. The formation of the ovum, shown in Part I. of this memoir (12 — 22.) to com- 

 mence before the existence of the ovisac, is completed in the number of its parts, 

 (though probably not matured) in or near the centre of this vesicle (Plate V. fig. 17.)- 



76. But if the ovum at this period is situated in the centre of the ovisac, by what 

 means is it supported there? At first this appears to be effected by an equable diffu- 

 sion of the peculiar granules of the ovisac throughout the fluid of this vesicle (Plate V. 

 fig. 17.)- Subsequently a peculiar structure arises partly for this purpose. 



77. The ovisac, as already said (24. 25.), acquires a proper covering or tunic sus- 

 ceptible of becoming highly vascular, and thus passes into the condition of what has 

 been denominated a Graafian vesicle. As this covering or tunic begins to form at the 

 period just referred to, viz. while the ovum is still in the centre of its containing cavity, 

 it will be proper in future to lay aside the term ovisac and to substitute that of Graafian 

 vesicle, the former being now no more than the lining membrane of the latter. 



78. The granulous contents of the Graafian vesicle now assume an amygdaloidal 

 appearance (Plate VII. fig. 48.), which seems to arise from a denser aggregation of 

 the granules in certain parts, and the occupation, by the fluid, of the spaces thus oc- 

 casioned ; in other words from a histological separation. If examined more closely, 

 however, the condition of the interior of the ovisac is not found to be really amygda- 

 loidal, as the cavities communicate with one another (Plate VII. fig. 49.). 



79. In a stage somewhat more advanced this separation is found to have proceeded 

 farther, the peculiar granules of the Graafian vesicle having arranged themselves so 

 as to constitute three distinct structures (Plate VII. fig. 50. g^, g'^, g^.). Some of 

 them are collected on the surface of the chorion, forming the tunica granulosa (g^), 

 which I have already described (64.) ; others constitute a structure, or rather an 

 assemblage of structures, consisting of a central mass, in which the ovum (in its gra- 

 nulous tunic) is contained, and of cords extending from the membrana granulosa to 

 this central mass {g'^), which will be now described ; and the third portion of these 

 granules is collected on the inner surface of the Graafian vesicle, where they consti- 

 tute the membrana granulosa (g^), this being, I apprehend, the manner of origin of this 

 membrane. See the sixth column in the table, page 304. 



The Retinacula. 



80. The central mass and cords of granules just mentioned (Plate VII. fig. 50. ^2.) 

 do not appear to have been hitherto observed. One of their uses will perhaps be 

 apparent from the figure now referred to. They obviously suspend the ovum, and 

 retain it in its situation in the fluid of the Graafian vesicle ; from which office, with 

 others to be hereafter mentioned, I propose to name these structures the Retinacula'\- . 



t Tlie granulous cords extending from the membrana granulosa to the central mass are probably much 

 more numerous at this period than represented in the figure ; but the minuter ones could not in this instance 

 be seen, nor are the minutest ever visible, from the exterior of the Graafian vesicle. 



