The fine structure of the portion of the parietal yolk sac which extends across the fetal surface of the rat placenta (which is the only portion of the parietal yolk sac which persists after the sixteenth day of pregnancy) was examined sequentially on the even numbered days during the second half of gestation. The placental parietal yolk sac was seen to consist of two cellular layers, trophoblast and endodermal epithelium, which were separated by a thick, amorphous membrane (of Reichert). The trophoblast attenuated with increasing gestational age and, on day 18, became perforated by fenestrations which were closed by diaphragmata. By day 22, the attenuated trophoblast was patently perforate, and maternal blood was exposed to Reichert's membrane. Since throughout gestation, the cells of the parietal endoderm formed a discontinuous layer, at all stages Reichert's membrane was exposed to the vitelline cavity. When ferritin and thorotrast were injected i.v., both separately and together, the former passed readily from maternal blood through Reichert's membrane and was phagocytosed by cells of the parietal endoderm. Thorotrast, however, did not cross Reichert's membrane. Since the rodent yolk-sac complex is known to serve as a placental route, selective permeability of Reichert's membrane may help regulate maternofetal exchange.