In considering the diversity of Eutherian mammalian placental structure, it is helpful to keep in mind that both phylogenetically and ontogenetically a functional yolk sac placenta precedes development of the chorioallantoic placenta. Usually the chorioallantoic placenta progressively displaces the area of contact of the yolk sac placenta with the endometrium. It is also closely applied to the endometrium, increasing respiratory efficiency but tending to decrease histotrophic nutrition. Catarrhine primates have minimal histotrophic uptake, using haemotrophic mechanisms throughout most of gestation. Rodents, by using partial or complete inversion of the yolk sac, have extensive regions available to histotroph in later pregnancy. Most mammals, however, have developed specialized regions of the chorioallantoic placenta for ingestion of uterine secretions, cell debris and erythrocytes. These regions vary from simple saccular areolae to more complex areolae such as the chorionic vesicles of prosimian primates to specialized haemophagous areas. All such structures are consistently lined by columnar cellular trophoblast. Examples of these structures, many of which can ingest both glandular secretions and erythrocytes (i.e. they are heterophagous), can be found in species whose definitive placenta is epitheliochorial. They are common but not universal in species whose definitive placenta is endotheliochorial and are even found in a few species with definitive haemochorial placentas. Restriction of phagocytosis to regions of columnar cells provides polarized cells appropriately oriented for ingestion, breakdown and transport, and limits the type of ingestion of both beneficial and potentially toxic materials to expendable individual cells.