The histogenesis and organogenesis of the human gonad in 12 embryos and 6 fetuses of ovulational ages 5 to 18 weeks was investigated by histological and ultrastructural examination, including observation of almost complete serial Epon-embedded sections of entire gonads of 10 embryos. This investigation revealed that the main constituent cells of the gonads are derived from the mesonephros, and that the coelomic epithelium is not involved in the formation of the main component at any stage. With the migration of the primordial germ cells into the gonadal ridge, the coelomic epithelium becomes stratified to form a moderate protrusion of the gonad into the coelomic cavity and the coelomic epithelial cells develop into short pillars which form cord-like structures, the so-called primary sex cords. Shortly afterwards, concomitantly with the development into the subsequent prominent protrusion of the gonad into the coelomic cavity, cells emerging from the mesonephros are incorporated into the gonad to form 'primordial sex cords'. At this stage, a stratified, pile-like arrangement of coelomic epithelium flattens into monolaminar or oligolaminar structures. In the testis, the 'primordial sex cords' differentiate into seminiferous sex cords by elaborating a surrounding basal lamina. In the ovary, these 'primordial sex cords' become displaced towards the peripheral regions of the gonad by the enlargement of these cords, as well as by the formation of the interstitium, or so-called medulla, at the base of the ovary; they differentiate into 'folliculogenous sex cords' which give rise to follicular cells.