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The cattle follicular oocytes matured for 26~28h in culture condition were examined at 4, 5, 6, 7, 8, 10, 12, 14, 16 and 18h after insemination with bull spermatozoa preincubated for 4.5h in the uter isolated from estrous hamsters. After further culture with spermatozoa for 4~18 h, 73~89% of the total oocytes had matured to the second metaphase. None of the follicular oocytes matured in culture, were fertilized 5h after insemination. But when the oocytes were examined at 6, 8, 10, 14 and 18h after insemination, 60, 73, 82, 80 and 87% of oocytes were fertilized, respectively. The majority of the fertilized oocytes had enlarged sperm head at 6h after insemination and a part of the fertilized oocytes begun to develop from enlarged sperm head to male pronuclear stage at 8h after insemination, and most of them developed to male and female pronuclear stage at 10h after insemination. The results suggest that the penetration of spermatozoa into the oocytes may occur earlier than 6h after insemination and development of their pronuclear stage may occur at 8h after insemination.
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