Figure 1a. Gonadal embryology. (a) Drawing of a 6-week embryo depicts migration of the primordial germ cells (red dots) from the yolk sac along the hindgut to the genital ridges. (b) Diagram of a midabdominal cross section shows incorporation of the primordial germ cells (red dots) into the primitive sex cords, which will continue to develop into either a testis or ovary. Unincorporated germ cells have the potential to form teratomas.