THE OVUM IN THE AMPHIBIA. 
189 
Recourse was had to a similar expedient to procure the seminal fluid from the 
male. Spallanzani had obtained the fluids by vivisection, from the seminal vesicles 
themselves. But there seemed more objection to the adoption of this mode with the 
male than the female, the certainty of much of the fluid being lost, independent of 
the severity of the operation. Indeed Spallanzani states, that he was never able to 
procure more than from two to three grains from a single individual. I therefore 
availed myself of a habit in the male frog, and which Spallanzani had previously 
noticed and taken similar advantage of in the Newt, to obtain the fluid in greater 
quantities than by the mode constantly adopted by that physiologist. When the male 
frog, like the Newt, is taken in the hand, or slightly compressed, at the season of pairing, 
a quantity of fluid is immediately passed. This consists chiefly of seminal fluid mixed 
with water expelled from the effect of the compression, or during the efforts to escape, 
as water is passed by other animals at the moment of capture. It abounds with 
spermatozoa in their most active state, and thus is fitted for experiment. It required 
therefore only to secure the limbs of the animal and compress it slightly, to obtain 
the fluid without severe injury. This ready mode was adopted on all occasions 
when the fluid was required, and the precaution taken always to examine a portion 
with the microscope, to be assured of its nature before employing it. Spermatozoa 
have never, during the season of pairing, been absent from it. At the end of the 
season they have been less abundant, and spermatozoal cells in greater proportion 
than at an earlier period. But in these cases I had reason to think that the chief 
part of the fluid consisted of water. It is probable that this was the case in the two 
instances of apparent absence of spermatozoa in the Toad, mentioned by Spallanzani*, 
and that the fluid did really contain spermatozoa, although few in number, and con- 
sequently easily overlooked, and that the ova were impregnated by these, and not by 
the fluid portion of the semen, as he appears to have supposed. 
On comparing the white surface of the yelk of the unimpregnated with that of the 
impregnated egg, whether the egg had been fecundated naturally or artificially, I was 
not able to detect any difference during the first twelve minutes. The changes went 
on in both, and appeared to be almost identical in each. But after the time speci- 
fied no further progress was perceptible in the unimpregnated ovum, which con- 
tinued to exhibit the same appearance for several hours. But the white surface of 
the impregnated egg became more and more changed, up to the time of cleavage of 
the yelk, when it was almost an uniform surface. 
These observations were afterwards repeated with similar results, and the conclu- 
sion to which they led was, that changes take place in the yelk from the period when 
the germinal vesicle disappears and the ovum leaves the ovary to the moment of its 
expulsion from the body, and which changes may proceed for some time afterwards 
quite independent of impregnation ; and that these have some reference to the evolu- 
tion of the central or embryo vesicle: possibly also that they do not cease imrne- 
* Dissertations relative to the Natural History of Animals and Vegetables, 1789, vol. ii. p. 151. 
