material anatomical entity placenta eutherian placenta Organ of metabolic interchange between fetus and mother, partly of embryonic origin and partly of maternal origin[GO]. The fetal portion of the placenta is known as the villous chorion. The maternal portion is known as the decidua basalis. The two portions are held together by anchoring villi that are anchored to the decidua basalis by the cytotrophoblastic shell. allantoic placenta