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.
At this period a little semicircular crescentic fold attached to the dorsal margin of the aperture of the vagina arises and forms the hymen, an organ which has always played such an important rôle in the fancy of all nations.
The upper blind ends of the Müllerian ducts, with their expanded funnel-shaped mouths, diverge and form the oviducts, or the Fallopian tubes.
D. The External Genitals
At the time when the urethra, the sexual ducts and the intestine still open into the sac-like tube, the so-called cloaca, there is distinguishable on the exterior surface of the body, corresponding to the position of the cloaca, a certain depression called the cloacal depression. When the intestine is separated from the cloaca by the septum, the later perineum, the exterior cloacal depression is cut into two, the anal and the urogenital depressions. Between the urogenital depression, later called the genital groove, exteriorly and the urogenital sinus interiorly, there is only a dividing membrane, the urogenital membrane which later on breaks through and transforms the entire sinus into a shallow depression, termed the vestibule.
CUT V.
Six stages of the development of the external genitals. Fig. 1 and 2, two in different stages; m and M, two male stages, f and F, two female stages after O. Hertwig.
pl, posterior limbs; clo, cloacal depression; gt, genital tubercle; gs, genital swelling; gf, genital fold; gg, genital groove; gp, glans penis; p, perineum; a, anus; pr, prepuce; sc, scrotum; r, raphe; cl, clitoris; su, entrance to sinus urogenitalis; lm, labia majora; ny, nymphæ; vv, vestibule of vagina.