DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY. 329 



■ 98. The ovisac has at first an elliptical or ellipsoidal form, becomes more spheri- 

 cal, and in certain Mammals somewhat tapered at one end (2. 34. 3.). 



99. 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 (4.). 



100. Myriads of ovisacs with their contents are formed that never reach matu- 

 rity (2.). 



101. Some of the ovisacs which do not reach maturity are situated in (he parietes 

 of Graafian vesicles in Mammalia, or of the corresponding structures in other Verte- 

 brata ; being sometimes formed in this situation, and sometimes included by the co- 

 vering which the larger ovisac acquires. The minute ovisacs so situated it is pro- 

 posed to denominate parasitic ovisacs (57-) • 



102. The ovisac is often found in a cavity proper to itself, with the walls of which 

 it has no organic union (6.). 



103. The granules forming the envelope of the germinal vesicle above referred to, 

 and subsequently found in the fluid of the ovisac, are very peculiar in their appear- 

 ance; they are generally of an elliptic form and flattened, highly transparent, contain 

 a nucleus and sometimes a pellucid fluid also, and are intimately connected with the 

 evolution of the ovum (8. 10.). These granules are present in largest quantity in the 

 ovisac of Mammalia; yet granules essentially the same exist at an early period in the 

 ovisac of Birds (32. 33.), and are sometimes met with in that of Fishes (34. 42.). 



104. A continual disappearance of ova, and the formation of others, are observable 

 even at a very early age (60.). 



105. The ovum of Mammalia when completely formed is at first situated in or 

 near the centre of the ovisac (75.). 



106. It is at this period supported in the centre of the ovisac by an equable diffu- 

 sion of granules throughout the fluid of the latter (76.). 



107. The ovisac about the same time begins to acquire its proper 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, therefore, 

 it is proper to speak, not of an ovisac, but of a Graafian vesicle {77')- 



108. The peculiar granules of the Graafian vesicle arrange themselves to form three 

 structures, viz. the memhrana granulosa of authors, and two structures not hitherto 

 described, one of which it is proposed to name the tunica granulosa, and the other, 

 which is rather an assemblage of structures than a single structure, the retinacula 

 (78. 79. 82— Note.). 



109. The tunica granulosa is a spherical covering proper to the ovum, and ex- 

 plains why the outer line in the double contour of the thick chorion, that is, the ex- 

 ternal surface of this membrane, 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 (64 — 7 1.). 



MDCCCXXXVIII. 2 u 



