Previous experiments demonstrated that a particular developmental relationship was critical in determining whether or not rat ova transferred asynchronously into the uteri of recipient rats would survive to term foetuses. The present experiments were designed to determine the fate of transferred ova, particularly at the time of implantation.
Ova that were I day younger than the uterus developed at the usual rate until the 5th day of pregnancy, but then degenerated rapidly and failed to implant. Ova that were 1 day older than the uterus delayed their development and did not implant until the uterus was ready for the implantation interaction. Neither control ova nor ova 1 day older than the uterus could implant on the 4th or 6th days of pregnancy.
It is postulated that in the afternoon of the 5th day of pregnancy, the ovum and the endometrium having independently attained a specific stage of development, the uterine environment suddenly changes, becoming detrimental to younger ova but stimulating to 5-day blastocysts in such a way that they elicit the decidual reaction, become attached to the endometrial epithelium, and begin the process of implantation.