Eggs from 79 Chinese hamsters, Cricetulus griseus, were examined at various times from before ovulation on the day of mating (Day 0) to implantation (Days 5-6). The animals were kept under reversed lighting (dark period 8 A M to 8 PM). Ovulation occurred between approximately 4 and 6 PM; mean number of eggs recovered per female was 7.6 2 0.2. Sperm penetration was almost complete by midnight on Day 0. Of 131 eggs collected on Days 1 4 , 102 (78% ) were fertilized; (females without fertilized eggs excluded). The midpiece and tail of the fertilizing sperm were found in the cytoplasm of only 9% of pronuclear or cleaved eggs, and remained in the perivitelline space of 74%; in the other 17% eggs the sperm tail and most of the midpiece were outside the zona pellucida or were missing at recovery. No polyspermic eggs were found.The first three cleavages took place at 20-26 hour intervals after ovulation. Eggs reached the blastocyst stage in the oviduct and entered the uterus more than 96 hours post-ovulation. Implantation occurred between Days 5 and 6.

Fertilization of eggs in vitro has been described in three mammalian species\p=m-\ the rabbit (Dauzier, laboratory mouse (Whittingham, 1968). Penetration by spermatozoa and pronucleus formation have also been observed in rat eggs, following dissolution of the zona pellucida by chymotrypsin (Toyoda & Chang, 1968).In preliminary attempts to fertilize eggs of the Chinese hamster, Cricetulus griseus, in vitro, no success was achieved using fresh spermatozoa, but 10% of eggs (8 of 80) were penetrated by spermatozoa which had previously been incubated with Syrian hamster cumulus clot. This treatment was used in a larger study in August 1968, as described here.Immature and maturing Chinese hamsters, 5 to 8 weeks old, were injected intraperitoneally with 5 i.u. pmsg (Equinex, Ayerst) followed 48 to 52 hr later by 10 i.u. hcg (A.P.L., Ayerst). Tubal eggs were recovered from females killed 15\ m=1/ 2\ to 16\ m=1/ 2\ hr after hcg and, from earlier studies, these were considered to be not more than 2 hr post-ovulation. Follicular eggs were obtained from females where ovulation had not begun or only a few follicles ruptured at this time, and from one female killed 14\ m=1/ 2\ hr after hcg. Syrian hamster cumulus clot was used for all pre-treatments of spermatozoa. It was obtained from superovulated mature females (Greenwald, 1962) killed on Day 4 of the cycle, 14^t o 15-J-hr after injection of 10 i.u. hcg; about half the cumulus from one oviduct was used for each preparation.Chinese hamster spermatozoa were stripped from the vas deferens into mineral oil and added to 1 ml of either Tyrode's solution or TC medium Ham FIO containing 1% bovine serum albumin. A drop of the suspension, 1 to 2 times the volume of the cumulus and containing approximately 4 to 6 million spermatozoa/ml, was then added to the Chinese hamster eggs or to Syrian hamster material for pre-incubation. Eggs for culture with the pre-treated spermatozoa were recovered from Chinese hamster females 2 to 3 hr later and added to the pre-incubation mixture. They were easily distinguished from the Syrian hamster eggs by their smaller size, as well as a different appearance of the * Present address : Zoology Department, University of Bristol, England. f 371

Summary. Rabbits