In embryology, the sex cords, (primitive sex cords or gonadal cords) are structures that develop from the gonadal ridges. After sexual differentiation, at day 49, the sex cords in males become the testis cords by the action of the testis-determining factor protein, which helps to develop and nourish the Sertoli cells. The testis cords are precursors to the rete testis. They play several different roles in the development of the male genitals.
In females the sex cords become the cortical cords, also called secondary cords. After further development they become the ovarian follicles.