Baboon placentae were studied which ranged in age from 13 to 40 days. Implantation in the baboon is superficial and a single, discoid, villous hemochorial placenta is formed. No chorionic villi develop peripheral to the placenta and there is no decidua capsularis. Trophoblastic tissue within the placenta is of two types, cyto‐ and syncytiotrophoblast. The chorionic villi are short and straight at 13 days but have begun to branch by 16 days. The villi consist of a mesenchymal core surrounded by a single layer of cytotrophoblast which is in turn surrounded by a layer of syncytiotrophoblast. Cytotrophoblastic cells from the distal tips of the villi spread over the maternal surface of the placenta and form a complete cytotrophoblastic shell against the uterine stroma. Maternal blood enters and leaves the intervillous space through clefts in the cytotrophoblastic shell which connect with arteries and expanded venules within the endometrium. A limited decidualization of the uterine stroma takes place.
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