The other villi atrophy, disappearing entirely by 16 to 18 wk. The spiral arterioles empty into the intervillous space so that maternal blood circulates around and through the latticework of villi, draining out through two or three venous sinuses associated with each spiral arteriole. The villi are divided into groups called cotyledons, each supplied by one or two spiral arterioles. At term, a placenta contains 10 to 20 cotyledons. Nutrients are transferred from maternal blood in the intervillous space, across trophoblast cells, through the fibrous core of the villus, and through the endothelial cells of the fetal capillaries to the fetal blood.
Wastes move in the opposite direction.
This arrangement is called a hemochorial placenta, because maternal blood is in apposition to fetal chorionic or trophoblastic tissue. The discoid placenta reaches its final form at 18 to 20 wk of pregnancy. The conceptus can be recognized as an embryo about 10 days after fertilization, when the ectoderm splits to form the amniotic sac. Then the primitive streak, which becomes the neural tube, begins to develop. Around day 16 or 17, the mesoderm thickens near the cephalic end, forming a central channel that ultimately becomes the heart and great vessels.