IT was shown recently¹ that sperms injected into the periovarian sac of the rat after ovulation did not begin to enter the eggs until four or five hours later. In the rabbit, too, sperms introduced into the Fallopian tubes shortly after ovulation seldom penetrated the eggs; but if sperms Were introduced a few hours before ovulation, the majority of the eggs Were later observed to be fertilized. Chang² reported similar findings in the rabbit, and also showed that sperms, injected into the Fallopian tubes after ovulation, were able to penetrate a larger proportion of eggs if they had first spent about five hours in the uterus of another rabbit.