The primordial germ cells are the common origins of spermatozoa and oocytes and thus represent the ancestors of the germline. Like all other somatic cells these are diploid and in human embryos can already be found in the primary ectoderm (epiblast) in the second week.
Emigration of the germ cells
In the third week, the primordial germ cells wander - in an amoeboid manner - from the primary ectoderm into the yolk sac wall and collect near the exit of the allantois. The primordial germ cells are now extraembryonal, lying in the endoderm and mesoderm of the yolk sac wall.
Facilitated through the cranio-caudal curvature and the lateral folding of the embryo, the primordial germ cells wander back into the embryo again between the fourth and sixth week. They move along the yolk sac wall to the vitelline and into the wall of the rectum. After crossing the dorsal mesentery they colonize the gonadal ridge. During their journey, but also while still in the gonadal ridge, the primordial germ cells multiply by mitotic divisions. Figure 4 provides a view of this journey. On the next page, a more detailed prospect is shown in figure 5.