The vole (Microtus agrestis) is an induced ovulator (Austin, 1957a; Breed, 1967) but the interval between copulation and ovulation is not accurately known. Ovulation occurs 9 to 12 hr after injection of pregnancy urine gonadotrophin (Austin, 1957a) but following natural mating, uncleaved eggs or eggs with one cleavage were found in Fallopian tubes 24 hr later (Austin, 1957b). The present study was undertaken to determine the time between coitus and ovulation and to ascertain the associated histological changes in the ovary.Virgin females were placed in cages with virile males, one pair/cage, and observed for 5 to 30 min, after which the pairs were separated. Three females were killed at various stages between 2 to 4 and 18 to 20 hr post coitum (p.c.). Ovaries were removed, sectioned at 7 μ, stained with haematoxylin and eosin and vesicular follicles
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