Fertility, ovulation and maturation of eggs in mares injected with HCG

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Abstract
Pony mares were observed from Jan.-Aug. for incidence of estrus, duration of estrus, length of the estrous cycle and for ovulation and fertility after injection of HCG [human chorionic gonadotropin]. From Jan to 15 May most mares showed estrus but the duration of estrus was quite variable and few mares ovulated in response to HCG. From 15 May to 17 Aug. estrous cycles were more regular and ovulation was induced within 40-50 h by an i.m. injection of 1500-5000 I.U. HCG. Pregnancy was established by 1 mating at a fixed time after HCG in 20 of 69 mares. Degenerate eggs were recovered from the oviducts of anestrous, recently ovulated, mated, unmated and pregnant mares. The 1st polar body was formed before ovulation in 2 eggs and had not formed in 2 recently ovulated eggs flushed from the oviduct. The 2nd polar body formed after sperm penetration 10-12 h after ovulation. After formation of pronuclei, the 1st cleavage division occurred at 20 h and the 2nd at 32 h after ovulation. Estrus was inhibited by progesterone administered by vaginal devices but occurred within 1-3 days in 12 of the 20 mares after withdrawal ofthe devices.

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