Human embryo at 6 weeks, illustration. At 6 weeks of gestation the embryo is enclosed by the amnion external to which is the chorion. Extending into the narrowing chorionic cavity is the long vitelline duct joined to the yolk sac. Villi cover the entire surface of the chorion, many of which later will form part of the placenta. From Kollmann, J. (1907) Handatlas der Entwicklungsgeschichte des Menschen, vol 1, Fig. 216. G. Fischer, Jena.