It has been reported in the literature that blastocysts during delayed implantation (here referred to as `dormant blastocysts') differ from blastocysts on Day 5 (spermatozoa in vagina = Day 1) of regular pregnancy (here referred to as `active blastocysts') as regards size, mitotic activity and fine structure. In the present study, it was attempted to determine whether these two types of blastocysts also differ in their physiological potential, i.e. whether they can survive under the same environmental conditions. The survivability was tested as follows: `active blastocysts' were transferred into `dormant uteri' (uteri during delayed implantation) and `dormant blastocysts' were transferred into `sensitive uteri' (uteri on Day 5 of pregnancy). In both types of transfers, the blastocysts developed into normal, live foetuses. The compatibility between the uterus and the blastocyst, if one is `active' and the other is `dormant', is discussed.
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