The primordial germ cells (PGC, cellulae germinales primordiales) arise from the epiblast. They are visible for the first time at S11 in the wall of the umbilical vesicle, and reach the endoderm through the caudal region of the primitive streak. They migrate along the allantois and the mesenterium of the hindgut and reach the gonadal, or genital, ridge medially from the mesonephric ridge at S14. During their migration, they proliferate by means of mitoses (mitosis) until there are a few thousand cells.
The primordial germ cells penetrate the thickened coelomic epithelium. At the same time, cells of the coelomic epithelium migrate into the gonadal blastema. Together with the germ cells, they form cylindrical cords ("primary sex cords", Satoh 1991). However, it is thought that these primary gonadal cords provide only a kind of scaffold for the protrusion of the gonads into the coelom. The primordial gonadal cords (chordae gonadales or "primordial sex cords", Satoh 1991) originate from cells of the mesonephros that migrate into the gonads and fail to reach the coelomic epithelium. Formation of the gonads requires both primordial germ cells and somatic cells. Somatic cells consist of cells of the coelomic epithelium, the local mesenchyme, and the mesonephros.
Sexual differentiation becomes recognizable from S18 for the male gonad and from S20 for the female gonad. The anlage of the gonads bulges into the peritoneal cavity. It remains attached to the mesonephric ridge through the mesenterium (mesogonadium). The cranial and the caudal parts of the genital ridge become the superior and inferior gonadal ligaments respectively.