The gonads (testes and ovaries) are derived from three sources: the mesothelium (coelomic epithelium) lining the posterior abdominal wall, the underlying gonadal ridge mesenchyme, and the primordial germ cells (precursors of spermatogonia or oogonia).

The primordial germ cells first appear in the wall of the yolk sac and in the 6th week and migrate along the dorsal mesentery of the hindgut to the genital ridges. Genital ridges form a thickened area comprising mesothelium and underlying mesenchyme on the medial side of the mesonephros.
Epithelium of the genital ridge proliferates and penetrates the underlying mesenchyme, forming the primitive sex cords.

1. Coelomic epithelium
2. Primitive sex cords
3. Primordial germ cells
4. Dorsal mesentery
5. Mesenchyme