178 MR. NEWPORT ON THE IMPREGNATION OF 



This disappearance does not, I think, take place at the instant the ovum is about to 

 leave the ovisac, but a short time before ; as I have found the ova of the Frog in one 

 instance more matured than in that already described, still contained in the ovisacs, 

 and yet most of them without any remains of the vesicle which I could identify as such, 

 and with the yelks containing a larger proportion of white substance, as in the Toad. 

 In a very few, however, the vesicle was still present, and exhibited the structure I have 

 described. Certainly, then, the vesicle is not burst at the moment the egg escapes 

 from the ovisac. There seems to be a short period of time between the disappearance 

 of the vesicle and the full maturity of the ovum, during which the yelk itself undergoes 

 some further change, and acquires the appearance noticed in the matured eggs of the 

 Toad, and in the most advanced of those of the Frog. No trace of the germinal vesi- 

 cle can be detected in any ova that have left the ovary and are contained in the cavity 

 of the abdomen, before entering the oviducts. I have examined many ova, both of 

 the Frog and Newt, from the abdominal cavity, but in every instance the vesicle has 

 entirely disappeared. In some specimens, which seemed to have been in the act of 

 escaping from the ovary when the animal was killed by immersion in spirit, I have 

 found in the place occupied by the vesicle an aggregation of white nucleated cells, 

 which, examined by the microscope, exhibited a close resemblance to those seen in 

 the interior of the vesicle. In the midst of these there has occasionally been one or 

 two of larger size than the rest, and which I have imagined to be the remains of the 

 germinal spot, and possibly the origin of the future embryo vesicle of the impregnated 

 ovum, an opinion, however, which I have not had the means of verifying, and I must 

 further state that I have failed to recognize these larger cells in ova that were free in 

 the cavity of the abdomen. Each of the three species of Newt, as well as the Frog, 

 have presented similar appearances in the germinal vesicle and ovum under similar 

 circumstances. 



Thus it is quite certain that the germinal vesicle disappears in the Amphibia before 

 the ovum enters the oviduct. I believe it does so in the interior of the yelk, not in 

 the centre, but nearer to the dorsal than to the future ventral or white surface ; and 

 not, as has been supposed, on the dorsal or dark surface, between the vitellus and the 

 vitelline membrane. This view is supported by the fact, that that portion of the yelk 

 which incloses the vesicle in an advanced stage of the ovum in the Frog is of a more 

 or less intense black colour, while the vesicle is perfectly white ; and that at a further 

 advanced stage, after the vesicle has disappeared, and its place is occupied by a col- 

 lection of white cells, the dark portion of the yelk still preserves its intense black 

 colour, except at the point that corresponds to the central canal, which then has a 

 leaden hue. PREVOST and DUMAS*, and also RUSCONI-J-, have mentioned that there 

 is a yellow spot at a corresponding part of the dark surface of the egg of the species 

 they have examined after impregnation, Rana esculenta ?, but these appearances must 



* Annales des Sciences Naturelles, torn. ii. p. 104. 



f De'veloppement de la Grenouille Commune, 4to. Milan, 1826. p. 9. 



