In the rat, two key reproductive phenomena—morula to blastocyst transformation, and implantation—are known to be affected by oestrogen/progesterone changes in the mother. Thus, experimentally imposed hormonal changes can prevent the transformation of morula to blastocyst (Dickmann, 1970, 1973) and delay and induce implantation (Nutting & Meyer, 1963; Mayer 1963). It has been axiomatic that the ovary is the source of oestrogen and progesterone necessary during the preimplantation period. The results of the present study suggest that the preimplantation embryo is an additional, perhaps indispensable, source of steroid hormones which influence the two phenomena mentioned.
Adult virgin female rats of the Holtzman strain, weighing 180 to 220 g, were mated with males of proven fertility. The morning of finding spermatozoa in the vagina was designated Day 1 of pregnancy. On Days 3, 4 and 5, rats were killed and the fertilized eggs were recovered according to the method described by Dickmann
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