In the human embryo, the ovary appears as a genital ridge about 1 month after fertilization. The primitive mesoderm gives rise to the mesothelial covering of the ovary and of the peritoneum, the so-called germinal epithelium from which epithelial ovarian cancers and primary peritoneal cancers are thought to arise. These cells arise along the medial and ventral borders of the mesonephros from which the mesenchymal tissues of the ovary are derived. The mesenchymal tissues give rise to the ovarian stroma. The germ cells originate from the primitive streak and migrate from the yolk sac endoderm toward the developing ovary. The primitive ovary is histologically unique and distinguishable from the primitive testis by 6 weeks after fertilization. The ovarian cortex develops by 8 to 9 weeks.