The rat blastocyst is dormant but viable during (1) lactational delay of implantation and (2) experimental delay of implantation. In the first situation the female is under progesterone dominance, but some oestrogen is also present (Grota & Eik-Nes, 1967); in the second situation the female is under the influence of progesterone only. It may be asked whether progesterone is necessary for blastocyst survival. Canivenc & Laffargue (1957) have concluded that the ovarian hormones are not necessary for blastocyst survival. The criterion for survival they used was the blastocyst's ability to form an implantation site (in their study the number of implantations was often greatly reduced). This criterion is open to criticism because an implantation site can be formed by a trophoblast shell devoid of inner cell mass (Tarkowski, 1962). Moreover, even if an implantation site contains an embryo, it may fail to develop into a term foetus. Cochrane & Meyer
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