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 fluid, 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 imme- 



* Dissertations relative to the Natural History of Animals and Vegetables, 1789, vol. ii. p. 151. 



