The ovary becomes differentiated later than the testis. The tunica albuginea does not become a prominent structure, while the primitive sex cords undergo medullary degeneration. In the 7th week, the surface epithelium proliferates and gives rise to a second generation of cords, the cortical cords. Later, these cords split into cell clusters that develop into follicles with granulosa cells surrounding a central oogonium (derived from the primordial germ cells). The oogonia subsequently proliferate and differentiate into primary oocytes. Thecal cells that surround the follicles derive from the mesenchymal stroma of the gonadal ridge.