Embryos developing from mouse blastocysts transplanted to the kidneys and testes of recipient mice were studied by light and electron microscopy. The yolk sac and Reichert's membrane of ectopic embryos were morphologically normal and had normal staining and histochemical properties. In electron micrographs Reichert's membrane was of low electron density and had a fine fibrillar structure. The cells of the parietal material within their ergastoplasm that closely resembled the membrane both in texture and electron density. The probability that the parietal yolk sac cells are responsible for the elaboration of Reichert's membrane in the mouse is discussed in relation to other work on the subject.

The occurrence of discrete eosinophilic bodies in transplants in which the development of the inner cell mass was abortive is reported. The light microscopic appearance and fine structure of these bodies revealed close similarities to the parietal yolk sac and to Reichert's membrane. It is concluded that they represent yolk sac cells embedded in an abnormal Reichert's membrane.