Attempts to induce polyspermy in the rabbit by delayed insemination and treatment with progesterone

Abstract Rabbits were injected with progesterone at various times before ovulation and inseminated at the time of ovulation (delayed insemination). When their eggs were examined 11–13 hours after ovulation, it was found that 15, 63, 73, and 61% of the eggs were penetrated following the injection of 4 mg progesterone at 24, 12, 6, and 0 hours, respectively, before ovulation as against 51% for control animals. Of the 213 penetrated eggs, 84% had two pronounclei with two polar bodies, 9% had three pronuclei without the second polar body, 3% had three pronuclei with the second polar body, 3% were unevenly cleaved, and 1% had two fertilizing sperm tails. These abnormalities were not correlated with the time of progesterone treatment. The formation of one or two pronuclei was frequently observed when aged, unfertilized eggs were cultured for 3–4 hours. It was concluded that the presence of three pronuclei following delayed mating in the rabbit is due to the formation of a pronucleus from the second polar body or to the fragmentation of the female pronucleus, rather than to polyspermy.