Parabiosis of rats on Day 1 of pregnancy with Day-1 pregnant females, non-pregnant females or male rats resulted in delayed implantation of ova. The delayed implantation was not caused by the surgery involved nor by the stress of restricted movement, for when surgery was performed without parabiosis, or when rats were sutured together without parabiosis, pregnancy proceeded normally. Delayed implantation did not occur when parabiosis was of the skin to skin type, but did occur when skin and muscles were joined. Parabiosis on Day 1 or Day 3 caused delayed implantation, but parabiosis on Day 5 did not. Parabiosis on Day 1 which was terminated after 24 hr did not result in delayed implantation, but 72 hr of parabiosis did. Experiments on adrenalectomized rats suggest that adrenal secretions are not involved. The decidual reaction of rats in parabiosis was inhibited. Ova recovered from rats in parabiosis were delayed in development, but some implanted normally when transferred to normal pseudopregnant recipients. Treatment of parabiotic rats with 0·1 μg of oestradiol on Day 4 of pregnancy resulted in normal implantation in some individuals.
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