THE EFFECT OF PREGNANCY ON IMPLANTED EMBRYONIC TISSUE.

1. Grafts of embryonic tissue obtained at operation and implanted in the mother, will grow well in case she no longer carries young. The growth is no more rapid than that in favorable nonpregnant aliens, but persists for a longer period without retrogression and results in a greater variety of tissues. The superiority of auto-transplantation over iso-transplantation is responsible for this fact. No evidence of a specific "growth-substance" peculiar to the pregnant state is furnished by the experiment. 2. When a mouse is implanted with embryonic tissue from her own uterus, and she still carries developing young, the fate of the grafts is very different from that just described. They are vascularized from the host but fail to grow or differentiate. Yet they do not die, and after pregnancy is concluded they may start to grow. The finding is strikingly like that noted by others of implanted tumor in pregnant hosts. It seems probable that some general factor affecting the growth of implanted tissues is here concerned.