The duration of the preimplantation period is not rigidly fixed in mice and rats. Although attachment of embryos to the uterine epithelium normally occurs late on the 5th day after fertilization, it may be delayed for days or weeks by concurrent lactation (naturally delayed implantation) or maternal ovariectomy (experimentally delayed implantation). It has long been known that the development of such 'delayed implanting' embryos is arrested at the blastocyst stage, and it has recently been shown that their overall embryonic metabolism is reduced. This depressed metabolic rate may be related to the ability of embryos to survive the prolonged free-living phase associated with delayed implantation.
How delayed implanting embryos are rendered metabolically dormant and then reactivated to implant at a later time is unknown. However, a comparison of changes in the rates of