Differentiation of the fetal gonad of Macaca fascicularis with special reference to the testis

The histogenesis of the fetal gonad of Macaca fascicularis was studied from the genital ridge stage in 24 fetuses (16 males and 8 females) aged 36 to 55 days. Observations were carried out using light and electron microscopy on serial sections of the specimens embedded in epon. The genital ridge was formed of a mesenchymal blastema covered by a peripheral epithelium. The first stages of differentiation (37 to 39 days) were the same in the testis and the ovary. We observed an antero posterior and centrifugal differentiation of the sex cords in the blastema from the peripheral mesenchyma of the mesonephric ducts at the anterior pole of the gonad. The mesonephric ducts and the peripheral epithelium contributed very little directly to sex cord formation (Pi. I). The separation of the newly formed cords was accelerated by vascular invasion from the mesonephros. Definitive testicular structure was acquired between 40 and 43 days. The presence of the peripheral testicular artery was a determinant of the growth of the tunica albuginea (Pi. 111). No large changes were observed in the ovary between 40 and 55 days (Pl. IV).