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. [Illustration: 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.] Before the urogenital sinus has opened to the exterior the mesenchym surrounding the urogenital depression exteriorly begins to thicken and produces an encircling elevation, the genital swelling. On the ventral side within this swelling appears a projection, the genital tubercle, which is thus surrounded by the genital swelling. The tubercle soon increases in size, so that the urogenital depression, now called the genital groove, becomes partly situated at its under aspect (Cut 5, Fig. 2). The lips of this genital groove thicken and form the two genital folds. All these four organs are common in both sexes and represent the asexual or bisexual state of the external genital organs. _E. The Male External Genitals_