Time Relations and Their Significance in the Ovulation and Penetration of Eggs in Rats and Rabbits

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Abstract
Observations were made on a total of 577 mated adult rats and 52 mated rabbits. The mean number of eggs recovered from 362 rats killed between 10.00 a.m. and 5.00 p.m. on the day of ovulation was 9.6/rat ([sigma]=2.2; range, 2-20 eggs), or4.8/oviduct ([sigma]l.9; range, 0-13 eggs). In rats mated under normal colony conditions, ovulation occurred between 12.00 midnight and 4.00 a.m. Ovulation of 6 eggs in any one rat took about 1 hour. Penetration by sperms occurred 2-4 hours after ovulation. Penetration of all eggs in any one rat took a mean time of 3.5 hours. Pronuclei were formed about 3 hours after sperm penetration. In rats kept under conditions of controlled illumination (light from 2.00 p.m. to 5.00 a.m.; dark from 5.00 a.m. to 3.00 p.m.), ovulation occurred between 11.00 a.m. and 3.00 p.m. Duration of the ovulation period (4 hours) and the time between the beginning of darkness and the commencement of ovulation (about 6 hours) were the same as in the rats mated under normal colony conditions. Similar also were the mean time for ovulation of 6 eggs in any one rat (3/4 hour), the interval between ovulation and sperm penetration (2.5-4 hours), the time required for penetration of all eggs in any one rat (3 hours), and the time between the penetration of the eggs and the formation of pronuclei (4 hours). Penetration of all the eggs in any one oviduct took about 2 hours. While penetrating the eggs, sperms spent a mean time of 0.5 hour in the perivitelline space. Groups of rats were mated at 3.00 a.m. (when ovulation was still in progress), at 7.00 a.m., or at 11.00 a.m. In each instance sperm penetration did not begin until at least 2 hours later. Such a period for the "capacitation" of the sperms is therefore probably normal and does not reflect merely a post-ovulatory alteration in the functional state of the tubes. In rabbits, sperm penetration began about 10 hours after mating, and all eggs were penetrated by the 13th hour. With coitus at the time of induced ovulation, penetrated eggs were not found until 5 hours later, although sperms were present at 4 hours. It appears therefore that capacitation of sperms in the rabbit takes about 5 hours. The delay of 2-4 hours between ovulation and sperm penetration in normally mated rats suggests that a change, possibly a final stage of maturation, must take place in egg membranes of the adult rat before sperms can enter the eggs. As no delay was observed in the rabbit, maturation of its egg is probably complete at ovulation.