Eight hundred and fifty-five 2- to 5-day-old ova were transferred from donor rats into the uterine horns of recipient rats on the 2nd through the 6th days of pseudopregnancy. The objective was to establish the lower, optimal and upper limits of development of both ovum and endometrium which were compatible with normal pregnancy. The lower and upper limits of development, 2 and 5 days respectively, proved to be the same for both ovum and endometrium. There was a striking tendency for ova the same age as, or one day older than, the corresponding stage of uterine development to survive more often than ova one day younger than the stage of uterine development. Data previously reported for the rabbit, mouse, and sheep suggest that ovular maturity may be of prime importance to implantation and survival of ova in these species also.
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