The primordial germ cells that eventually from , the oogonia and spermatogonia originate in which of the following
The primordial germ cells are first seen in the endodermal lining of the wall of the yolk sac (derived from the hypoblast) at the end of the third or beginning of the fourth week in the region of the allantois. During embryonic folding, the dorsal part of the yolk sac is incorporated into the embryo as the primitive gut. The primordial germ cells subsequently migrate along the dorsal mesentery of the hindgut and into the gonadal (genital) ridge by week 6. The primary sex cords grow into the mesenchyme underlying the ridge, and the primordial germ cells become incorporated into the primary sex cords. The chorion is the outermost fetal membrane and is composed of extraembryonic somatic mesoderm, cytotrophoblast, and the syncytiotrophoblast. It is divided into the chorion frondosum, where the villi form and proliferate. and the smooth chorion, also known as the chorion laevae. The following diagram illustrates the arrangement of the fetal membranes.
Dorsal mesentery of the hindgut
Gonadal ridge
Endodermal lining of the yolk sac
Primary sex cords of the developing gonad