The timing of sperm penetration, pronuclear formation and syngamy was examined in twenty-four gilts in which ovulation had been controlled precisely by means of an injection of hcg given during prooestrus. Animals were mated at the time of ovulation and eggs recovered at autopsy exactly 3, 4, 5 or 6 hr later.
Twenty of the twenty-four animals (83 %) yielded some penetrated eggs, a total of 234 eggs being examined. The mean number of spermatozoa attached to or within the zona pellucida increased progressively from 6·8 (range 1 to 25)/egg at 3 hr to 109·2 (range 1 to 466) some 6 hr after mating. Likewise, the proportion of eggs classified as fertilized increased from 23·7% at 3 hr to 71·9% by 5 hr after mating. Developing pronuclei were observed at 4 hr and apposing pronuclei within 5 hr of mating, but most of the penetrated eggs examined at these intervals were in anaphase or telophase of the second meiotic division.
The results emphasize that under conditions of induced ovulation, a population of capacitated spermatozoa first becomes available in the Fallopian tubes of oestrous pigs within 2 to 3 hr of semen deposition by way of the cervix. It is suggested that the sequence of events leading to sperm penetration may be accelerated following injection of hcg during pro-oestrus and mating at the time of ovulation.