AbstractPortions of the labyrinth or villi of placentas from late pregnancy from nine species in four orders of mammals were examined with the electron microscope. Pronounced patterns of layering of the trophoblast were found in these placentas which were all of the hemochorial type. The laboratory rat, laboratory mouse, hamster, and deer mouse have three layers of trophoblast between the maternal blood space and fetal vessels (hemotrichorial), the rabbit two layers (hemodichorial), the guinea pig and chipmunk one layer (labyrinthine hemomonochorial), and the human and armadillo one layer (villous hemomonochorial).The outer layer of trophoblast of the hemotrichorial placentas (the layer next to the maternal blood) is cellular, but the next two layers are apparently syncytial and are closely apposed to one another. The outer layer of the rabbit placenta is syncytial, while the inner layer contains some pockets of cells. In all of the hemomonochorial placentas examined, the continuous layer of trophoblast was syncytial. It was found that the surface layer of trophoblast of all the placentas studied is rich in granular endoplasmic reticulum, whereas in subsequent layers this element is less abundant. All of the placentas show both surface and basal modifications of trophoblast, but caveolae (pinocytotic vesicles) were found to be most commonly located in crypts, pockets, or other regions somewhat removed from the surface. It is suggested that an area of relative stasis of maternal plasma may be important for absorption of some substances by the placenta.