Human embryo at 8 weeks, illustration. At 8 weeks of gestation most of the amnion and chorion have fused. At the embryonic pole closest to the umbilicus, the chorionic villi have grown in number and size forming the chorion frondosum (bushy chorion). This forms the embryonic/foetal part of the placenta. Villi on the abembryonic pole of the chorion have degenerated, the chorion here is smooth (chorion leave) and does not participate in placental functions. The yolk sac and vitelline duct will degenerate by the third month. From Kollmann, J. (1907) Handatlas der Entwicklungsgeschichte des Menschen, vol 1, Fig. 218. G. Fischer, Jena.