ABSTRACT
Yolk sacs from rat placental membranes taken on the 14th or 15th day of pregnancy grew and differentiated in a remarkable manner when transplanted into the mother’s omentum. A variety of tissues developed within them, including mucus-secreting cysts, epidermoid cysts, cartilage, and bone. The mucus-secreting cysts were surrounded by smooth muscle and closely resembled intes-tine; they probably arose from the endoderm and mesoderm of the yolk sac. The epidermoids could have originated either from amnion remnants included accidentally in the transplant, or from metaplasia of the yolk-sac endoderm. It is obscure whether the cartilage and bone developed from host or graft. Transplants were rarely successful when removed from rats at the 11th or 18th day of gestation.
Similar results were obtained when the hosts were other stock rats, either normal or ovariectomized females or males.
The grafts were well tolerated by the hosts and homograft reactions did not occur. Members of the rat stock used would not tolerate adult skin homografts in the omentum, but transplants of foetal skin and bone grew and differentiated well. It is concluded that the tolerance of yolk-sac membranes is not a charac-teristic peculiar to placental tissue, but is part of a wider phenomenon of toler-ance to foetal tissues in general.
