. DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY. 827 



terminate pressure appears to be penetration through the raembrana granulosa, when 

 the latter has not been previously removed. Another effect, probably in all instances, 

 is the attenuation or removal, at one point, of the proper granulous tunic of the ovum 

 (69.). By continued pressure, also, the retinacula may contribute to attenuate that 

 part of the inner membrane (formerly the ovisac) of the Graafian vesicle in contact 

 with which they lie-}-, and thus promote the expulsion of the ovum from the ovary. 



89. All those who have been engaged in researches of this kind, must have noticed 

 that on the bursting of an ovisac under the compressor, the ovum and a mass of gra- 

 nules are among the parts escaping first. This has been mentioned as remarkable. 

 I think it will appear less so if the situation of the ovum as seen in most of the figures 

 in Plate VII. be considered. 



90. The whitish or yellowish white speck visible with the naked eye from the ex- 

 terior of the ovary in the Dog, which led to Baer's discovery of his " ovulum" (ovum) 

 of Mammalia, appears to be in this animal the tunica granulosa rather than the 

 ovum. In some others in which such specks are visible from the exterior of the 

 ovary, the central portion of the retinacula is also seen. This I find to be the case 

 in the Rabbit and the Ferret, and in minute Graafian vesicles of the Ox. In the 

 Rabbit I have with a good pocket lens discerned, from the exterior of the ovary, 

 several of the hand-like portions of the retinacula ; and this I find possible in minute 

 Graafian vesicles even of the Ox. 



91. I have now gone through a minute description of the parts concerned in pro- 

 ducing those appearances denominated by Von Baer the "cumulus" and "disc," as 

 figured in Plate VII. fig. 62., and Plate VIII. fig. 80. If these figures, taken from 

 that author J, be contrasted with g^ of my own figures in Plate VII., and particularly 

 of fig. 55., it will be obvious that the "cumulus" of Baer is made up of what I have 

 called the tunica granulosa, and the central portion of the retinacula ; and that the 

 band-like portions collectively of what I have called the retinacula, mainly contribute 

 to present the appearance denominated the "flat disc" by Baer. 



is rendered probable by the fact, that when gentle compression has in some degree displaced this vesicle, it re- 

 sumes its previous situation as soon as the pressure has been removed. This fact was observed by Professor 

 Baer, but from not having sees the true form of the structures here called the retinacula, he was not in a con- 

 dition to explain it. 



One of the figures now referred to, from the Dog (Plate VIII. fig. 66.), presents four ova in one Graafian 

 vesicle, which number, as well as three, I have repeatedly met with in this animal. Professor Valentin has 

 also figured three in one Graafian vesicle of the Dog (Dissertation by Beknhakdt, "Symbolae," &c., fig. viii.). 

 Two ova are very frequently present in the same Graafian vesicle in this animal; but in the Ferret {Mustela 

 Furo) this is still more frequently the case. In the last-mentioned animal indeed I saw in many instances 

 three, and not rarely four, ova in one Graafian vesicle. When this is the case, it is not unusual to find one or 

 two ova apparently defective. To explain the existence of a plurality of ova in one Graafian vesicle, we must 

 suppose that in such instances the membrane of the ovisac forms around, and thus includes in one cavity, the 

 granulous envelopes of a corresponding number of germinal vesicles (23.). 



t Von Baer, without having seen the peculiar structure of the retinacula, conjectured that this effect might 

 be produced by the "ovulum" (ovum) on the parietes of its Graafian vesicle. 



X Figs. ix. and xii. of Baer, Lettre, &c. 



