Ovariectomy of hamsters on day 1 or day 4 of pregnancy without steroid replacement resulted in complete failure of implantation. This was, however, induced by daily subcutaneous (s. c.) injection of either 1 or 2 mg progesterone. Although implantation could occur in the absence of oestrogen, greater numbers of eggs implanted in its presence after ovariectomy on day 1. Oestrogen administration (1 μg/hamster/day s. c. of oestrone) also increased the proportion of embryonic development to day 14 in the ovariectomized, progesterone-treated animal.
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Harper et al. (1966) studied this question.