THE OVUM IN THE AMPHIBIA. 175 



Mammalia does not disappear, as believed by PURKINJE*, in Birds and Amphibia, by 

 bursting during the generative act, and pouring its contents into the germinal layer 

 of the fecundated ovum. Neither, as supposed by BAER-{~, by being urged forwards 

 and burst between the vitellus and its membrane before fecundation. Nor, as stated 

 by RATHKE|, with reference to the Crustacea, by disappearing while still within the 

 ovary. Or, as supposed by WHARTON JONES , in Birds and Amphibia, by approaching 

 the surface of the yelk, and by the giving way of the coats of the vesicle, and the 

 effusion of its contents on the surrounding surface of the yelk. Or, further, as 

 believed by BISCHOFF ||, by disappearing at the time of exit of the ovum from the ovary. 

 On the contrary, Dr. BARRY has stated that the germinal vesicle returns to the centre 

 of the ovum, and the germinal spot to the centre of the vesicle, before the ovum leaves 

 the ovary, and that these do not become dissolved, but only are changed in character 

 by a process of cell development within them, which ends in the production of two 

 cells in the centre of the yelk, which are the foundation of the body of the future 

 embryo. 



Perhaps what I am about to mention may assist us to reconcile or correct the views 

 of this embryologist, as well as those of the authorities he has quoted. It is well 

 known that if the ovum of the Frog is examined before the yelk has attained to one- 

 half of its ultimate dimensions, the germinal vesicle is distinctly visible as a large cir- 

 cular body near the centre of the yelk, apparently granular in its interior, and more 

 opake than the yelk itself. In the centre of the vesicle its nucleus, the germinal 

 spot, is then equally distinct. If the ovum is examined when it has nearly acquired 

 its full dimensions, the vesicle is still found to exist, but, as compared with the size 

 of the entire ovum, is relatively smaller than at earlier periods, and is recognized 

 with more difficulty, owing chiefly to the yelk cells having both increased in number 

 around it, and also acquired a darker colour. It is equally well known that all 

 appearance of the vesicle is lost in the Amphibia, as in Birds, before the ovum is pre- 

 pared for fecundation ; but as to the way in which it disappears, observers are not 

 agreed, or even as to the time. 



Structure of the Germinal Vesicle. Having collected a number of frogs that had 

 recently left their hybernacula, and had not yet paired, I placed some in spirit for 

 examination. On dissecting them afterwards, I found that in some the ova had 

 nearly reached maturity, but had not left the ovary (Plate XIV. fig. ^ p). The yelks /<3. 

 being rendered firm by the spirit, I was able, by gentle pressure, to break open some 

 of these beneath the microscope, without diffluence of the contents. The aggregated 

 yelk cells were then seen to consist of two kinds ; the one dark-coloured, which form 



* Symbols ad Ovi Avium Historiam ante Incubationem, 1825 ; and Article "Ei," Encyclop. Worterbuch, 

 Bandx. p. 112, 1834. 



t " Lettre sur la Formation de 1'CEuf," in Breschet. Repertorium, 1829. 



J Untersuchungen ueber die Bildung und Entwickelung des Fluss Krebses, fol. Leipzig, 1829. 



Philosophical Transactions, 1837, part 2. || R. WAGNER'S Lehrbuch, &c., 1839. 



