After the genital ridge had made its appearance, columns of cells begin to grow down into the substance of the Wolffian ridge. The columns are composed of two kinds of cells, small epithelial cells and large spherical cells, known as sex-cells. Two regions may be now recognized in the ridge, the rete-region and the sex-gland region. The cell-columns of the rete region are termed “rete-cords,” and the cell columns of the sex-gland region are called “sex-cords.” The sex-cords unite to form a complicated network and the rete-cords grow backward to the Wolffian ridge. They then develop a lumen and send off branches to the sex-cord reticulum. The genital portion of the Wolffian body persists in the male in its entirety and serves as the efferent ducts of the testis. They open into the upper part of the Wolffian duct. The latter is retained complete. The portion nearest the testis is thrown into coils and forms a part of the epididymis, the remainder is converted into the vas deferens and the ductus ejaculatorius and the lateral outpouching of the wall, the vesicula seminalis. The Müllerian ducts disappear completely in the male. Only the lower ends of the ducts fuse to form the sinus pocularis, or utriculus prostaticus. _C. The Internal Female Sex Organs_ In the female the Wolffian body and ducts degenerate. The remainder of the body is known as the parovarium, an organ without any apparent function, while the remainder of the ducts are designated the ducts of Gartner. The ovary is produced from the asexual stage by the following metamorphosis. The mesothelial cells on the peritoneal surface of the sexual gland change into the germinal epithelium and form the so-called egg-columns or sexual cord which represent the primitive ova. [Illustration: CUT IV. _Vertical cut through male and female embryos, Schema; Fig. 1, male; Fig. 2, female._ a, allantois duct; b, bladder; u, urethra; ur, ureter; su, sinus urogenitalis; sp, sinus pocularis; cd, cloacal depression; ad, anal depression; t, testicle; hy, hyatide; e, epididymis; vd, vas deferens; Md, Müllerian duct; de, ductus ejaculatorius; k, kidney; i, intestine; va, vagina; ov, ovary; pa, parovarium; Ft, Fallopian tube; Wd, Wolffian duct; ut, uterus.] At the caudal end, the Müllerian ducts fuse together into one, the walls, along the entire line of the union, degenerate, and the two ducts thus form a single duct, the later vagina and uterus. Until the fifth month there is no distinction between vagina and uterus, the two organs form a single sac-like structure. At the beginning of the fifth month, a circular ridge in the wall of the sac makes its appearance and marks the division between the vagina and the uterus. When the lower portion of the two Müllerian ducts have fused to form a single canal, the utero-vaginal sac, the lumen of the vagina is still obliterated, being filled with epithelial cells. By the breaking down of the central epithelial cells, the cavity is established.