Fertilization and early development in the Chinese hamster, Cricetulus griseus

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 AM to 8 PM). Ovulation occurred between approximately 4 and 6 PM; mean number of eggs recovered per female was 7.6 ± 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.