352 GENERATION. 



four occasions, at intervals of one or two days. Sixty-two 

 days after the first injection, the bitch had four pups, which 

 resembled the male as well as the female. "With the semen 

 of the same dog, he attempted to fecundate two cats in heat, 

 but was unsuccessful, although the experiments were made 

 in the same way as with the bitch. 1 It is not easy to explain 

 why the spermatozoids from the dog could not penetrate the 

 vitelline membrane and fecundate the ova of the cat, but 

 this seems to be impossible in animals so widely different. 



As to the mechanism of the penetration of spermatozoids 

 to the vitellus, we can only refer to the micropyle discovered 

 in the ova of fishes and mollusks, which we have already de- 

 scribed. 2 In the ova of the Nephelis, a small species of leech, 

 Robin has seen spermatozoids, to the number of several 

 hundreds, penetrate the vitelline membrane, always at one 

 point, continuing their movements upon the surface of the 

 vitellus. " Almost always, when the penetration has ceased, 

 a bundle of spermatozoids are arrested in the micropyle." 8 

 We had an opportunity of witnessing a demonstration of 

 these phenomena by Prof. Robin, in 1861, in the ova of the 

 Limneeus stagnalis, and actually saw a spermatozoid half-way 

 through the vitelline membrane. According to numerous di- 

 rect observations, the spermatozoids move actively around the 

 ovum, collect toward a certain point, and there penetrate the 

 vitelline membrane. Coste, and many other observers, whom 

 it is unnecessary to quote, have seen the spermatozoids within 

 the vitelline membrane, in the ovum of the rabbit ; 4 and, 

 more recently, Weil has seen spermatozoids wedged in the 

 substance of the zona pellucida, has added blood to the speci- 

 men under observation, and has restored the movements of 

 the spermatozoids while in this position. He has also seen, 



1 SPALLANZANI, Experiences pour servir d Vhistoire de la generation, Pavie, 

 1797, p. 310, et seq. 



2 See page 290. 



8 ROBIN, Memoire sur les pJienomenes qui se passent dans V ovule. Journal de 

 la physiologie, Paris, 1862, tome v., p. 80. 



4 COSTE, Developpement des corps organises, Paris, 1859, tome ii., p. 103. 



