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 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, which 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 from 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 
f Muller’s Archiv, 1834. 
* Loc. cit, 
X Philosophical Transactions, 1839, 1840. 
§ Entwickeliingsgeschichte des Kaninchen-eies, 1842. 
j| Loc. cit. part 2. p 111. 
