The normal periodicity of the oestrous cycle in sheep is not significantly altered when the formation of additional corpora lutea is induced during the luteal phase by the administration of gonadotrophic hormones (Casida, Dutt & Meyer, 1945). It has been established by Inskeep, Oloufa, Pope & Casida (1963) that such artificially induced corpora lutea become fully functional, but regress prematurely at the time of involution of the naturally formed corpus luteum. The life-span of induced corpora lutea is even more restricted, however, when they are formed in ewes that have previously been treated with exogenous progesterone for an extended period of time (Inskeep, Howland, Pope & Casida, 1964a). As a result of their studies Inskeep et al. (1963, 1964a, b) concluded that the life-span of artificially induced corpora lutea is dependent upon the level of
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