RESUMO
Twenty four human embryos and foetuses of normal appearance were collected and used. Their ages were estimated, approximately in weeks, which divided into 7 groups included the 5, 6, 7, 8, 9, 28, 36th, weeks. The specimens were fixed immediately in 10% formal saline. The paraffin sections were stained with haematoxylin and eiosin and toulidine blue stains to be examined by light microscope and photomicrographed.The results of the present study confirmed the extragonadal origin of the primordial germ cells from the wall of the definitive yolk sac. Their local migration through the dorsal mesentery was observed, but most of them were seen carried by the blood stream through the vitelline veins to the posterior cardinal veins to reach and populate the gonadal rudiment, in the [5th] week embryo, where they settled among the undifferentiated mesenchymal cells underneath the single layered, cuboidal cells of coelomic epithelium which just, began their mitotic activity, In the 6th week embryo, the gonads enlarged and bulged into the coelomic cavity. The coelomic epithelial cells were proliferating into several cellular arrays forming the primitive tunica albuginea containing scatterd [still migrating] large size primordial germ cells, which became clumped with some of these epithelial cells, as presertoli cells, to form the primitive sex cords. In the 7th week embroys, some gonads were differentiated as testis by acquiring certain features included Ihe separation of the coelomic epithelium into a single layer of flattened cells, as well as the more elongation and branching of the primitive sex cords. Meanwhile, other gonads that do not show similar changes were considered as undifferentiated ovaries in female embryos. By the 8th week some cells from the primitive sex cords were segregated and spreaded to the surrounding interstitial blastoma. They were supposed to develop into the secreting "Leydig cells". By the 9th week the sex cords were more elongated and extended towards the mesonephric duct by solid cellular cords of the rete testis. By the 36th week, the testes were enlarged and reached to the scrotum.The fibrous tunica albugina was developed. The seminiferous cords were more thick and convoluted with narrow slit like lumena. The intenstitial blastema had wide fibrous stroma with scattered collections of intersititial cells between the seminiferous cords. The epididymis was more elongated and convoluted.The ovaries in the 28th week foetus were covered by a very attenuated single epithelial layer without tunica albugina. The cortex was fomed of fibrous spaces contained the oocyte surrounded by the follicular cells while the medulla was formed of fibrous stroma rich in blood vessels