184 MR. NEWPORT ON THE IMPREGNATION OF 



discovery of the cleavage of the yelk as a process of the fecundated egg ; to Rus- 

 CONI*, BAER-f-and others, its full exemplification in the Amphibia ; and to BARRY J and 

 BISCHOFF^ its detection and elucidation in the Mammalia. 



The agent immediately concerned in these changes is believed to be the embryo 

 vesicle and its progeny, produced after the disappearance of the germinal vesicle. 

 But it is yet uncertain what is the origin of the embryo vesicle, or whether it exists 

 in the unfecundated ovum. As cleavage of the yelk certainly is not the result of the 

 disappearance of the germinal vesicle, whicli disappears from all ova of the Amphibia, 

 whether they are afterwards impregnated or not, I was desirous, at the commencement 

 of my experiments on impregnation, to learn/rom direct observation whether the un- 

 fecundated ovum ever passes through any stage of cleavage ; since the ascertainment 

 of the fact in the negative would be an important test in the experiments I was about 

 to make. For this purpose it was necessary to collect many pairs of frogs at the 

 proper season, and when from symptoms which are soon recognized, it was found 

 they were about to cast their ova, to wait patiently, perhaps for many hours, for the 

 result, in order that the exact condition of the ovum, impregnated naturally, should 

 be first ascertained. SWAMMERDAM long ago remarked that the spawning of the frog 

 takes place very rapidly "by a single effort ||." It is often completed, as I have 

 found in the English species, in a few seconds, and usually in less than a minute, 

 during which the male impregnates them, so that if the animals are not closely 

 watched the opportunity of observing the earliest appearances of the ovum is lost. 

 Having noted the condition of the impregnated ova of several pairs of frogs within 

 the first few minutes after spawning, I found those of different individuals vary much 

 with respect to the white or inferior surface, and exhibit appearances that may readily 

 be mistaken for the breaking up of a vesicle on the surface. This appearance is due to 

 a more or less complete state of maturity of the eggs of different broods, and according 

 as their spawning has been retarded or hastened. The peculiarities are the most 

 marked in the least matured, the white surface of the egg being the last completed 

 part, and forming the base of the egg in the ovisac (fig. 5). Having noticed the 

 appearances of the eggs when impregnated naturally, I was enabled to compare them 

 with others impregnated artificially, and these with some of the same brood not 

 impregnated. 



Immediately after the frog has spawned the ova form a close rounded mass, which 

 at first is scarcely so large as a walnut. They then seem to consist almost entirely 

 of dark-coloured yelks with thin gelatinous envelopes. The form of the egg is then 

 somewhat oval, with the white portion a little more conical than the dark and 

 differing slightly in different ova. In some there is a dark spot in the centre of the 

 white, that looks like a depression or cavity, or perhaps a vesicle. I am not certain 



* Loc. cit. f MULLER'S Archiv, 1834. 



t Philosophical Transactions, 1839, 1840. 



Entwickelungsgeschichte des Kaninchen-eies, 1842. || Loc. cit. part 2. p 111. 



