Removal of the conceptuses (decidua+embryos) from the uterus of the guinea-pig between the 9th and 15th days after mating allowed the recurrence of oestrus at, or only slightly after, the time it would have occurred had not fertile mating taken place. The transfer of a single 6-day blastocyst to the uterus of unmated cycling animals resulted in pregnancy in three of six animals in which the cycles of the host and donor were synchronized. Normal development was never observed after transfer of 9- to 11-day implanted conceptuses (without associated decidua) to the uterus. Blastocysts and 9- to 10-day implanted conceptuses transferred to the spleen developed in eight of thirteen animals. The ectopic placental tissue did not prevent the normal recurrence of oestrus. Grafts developing from a single 11- to 12-day implanted conceptus in the spleen maintained the corpora lutea and delayed oestrus until after the 20th day in thirteen of twenty-four animals. The guinea-pig placenta appears to produce a systemically-active substance capable of neutralizing the luteolytic abilities of the uterus. This anti-luteolytic hormone probably acts only between Days 12 to 25 after mating.
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