OVULATION, FERTILIZATION, AND EARLY CLEAVAGE IN THE HAMSTER (Mesocricetus auratus)

SYNOPSIS The mean number of eggs ovulated was found to be 10.0 eggs per hamster, range 3 to 17. The fertilization rate was 85.7 p.c. In eggs recovered shortly after ovulation the mean overall diameter was 117.8μ, the thickness of the zona pellucida was 11.6μ, and the diameter of the vitellus was 73.2μ in unpenetrated eggs and 70.9μ in penetrated eggs. The first polar body always persisted until after ovulation. When the second maturation spindle could be identified it was always in metaphase in unpenetrated eggs examined soon after ovulation. The hamster egg has an unusually large perivitelline space. The zona pellucida dissolved in acid solutions at pH 2.8 and lower; it was readily digested by trypsin solutions, the zona from penetrated eggs being a little less resistant than that from unpenetrated eggs. The zona may undergo expansion during cleavage and was often lacking from eggs in the 4‐cell and later stages. When hamsters were kept from birth or weaning under controlled illumination (light from 4.0 p.m. to 7.0 a.m., dark from 7.0 a.m. to 4.0 p.m.), ovulation occurred chiefly between 2.0 p.m. and 4.0 p.m. There was a delay of approximately 2 hr. between ovulation and spermatozoon penetration. With natural lighting (July to September), nearly half the eggs recovered at 7.0 p.m. were in stages immediately preceding the first cleavage. On the next day (day 2) all fertilized eggs were 2‐cell, on day 3 they were 3‐ to 8‐cell, and on day 4 they were morulæ or early blastocysts. The process of fertilization is briefly described and compared with that seen in rat and mouse eggs. The male and female pronuclei resemble each other in size and form and are often indistinguishable. When one pronucleus is larger than the other it is probably the male. In nuclear and nucleolar dimensions the hamster egg resembles the mouse egg rather than the rat egg. In 519 penetrated eggs the incidence of polyspermy was 1.08 p.c. Not one of the penetrated eggs, however, had a supplementary spermatozoon. The zona reaction in the hamster egg is therefore very‐rapid and may constitute the only protection against polyspermy. Six eggs presenting anomalous features are described. Results are discussed in relation to previous observations on the hamster and other species.