Polyspermy can be produced experimentally in pigs by delayed mating, which results in the fertilization of aged eggs (Pitkjanen, 1955; Thibault, 1959; Hancock, 1959; Dziuk & Polge, 1962; Hunter, 1967). A high incidence of polyspermy has also been observed in pigs following induced ovulation and insemination during the luteal phase of the oestrous cycle (Hunter, 1966). These observations suggest that the endogenous level of progesterone at the time of fertilization might affect the block to polyspermy in pig eggs, since, under both conditions cited, developing or fully functional corpora lutea would be present in the ovaries at the time of sperm penetration. The present study was conducted to determine the frequency of polyspermy in pigs injected with progesterone at various intervals before ovulation and fertilization.
The experimental animals were thirty-six mature, crossbred, Large White × Essex,