Growth of mouse embryos implanted in the subcutaneous tissue of recipient mice

Eight-days-old mouse embryos were transferred to the subcutaneous tissue of the dorsal skin of host mice. A high rate of embryos developed into hemorrhagic (HN) or nonhemorrhagic nodules (NHN). The latter had trophoblastic cells as well as embryoblastic derivatives whereas HN contained almost only trophoblastic cells. At least two kinds of trophoblastic cells were present in NHN: small cells and large cells similar to trophoblastic giant cells. In HN most trophoblastic cells arranged themselves into a network whose meshes contained host blood. Although embryoblastic derivatives such as endoderm and Reichert's membrane were apposed to host connective tissue cells, trophoblastic cells were always surrounded by collagen fibrils or by a layer of an amorphous material.