. Chordate anatomy. Chordata; Anatomy, Comparative. THE UROGENITAL SYSTEM 323 MuUerian ducts unite to form uterus and vagina. The anterior portions form the uterine tubes. External Genitals. The external genitals of the two sexes, like the gonads, have similar beginnings. Slight differences, however, quickly make their appearance. In an 8 mm. embryo, a rounded eminence, the genital tubercle, develops between the tail and the umbilical cord. Along its caudal surface extends a shallow urethral groove bordered by urethral folds, the inner genital folds. Labial or scrotal swellings, the outer geni
. Chordate anatomy. Chordata; Anatomy, Comparative. THE UROGENITAL SYSTEM 323 MuUerian ducts unite to form uterus and vagina. The anterior portions form the uterine tubes. External Genitals. The external genitals of the two sexes, like the gonads, have similar beginnings. Slight differences, however, quickly make their appearance. In an 8 mm. embryo, a rounded eminence, the genital tubercle, develops between the tail and the umbilical cord. Along its caudal surface extends a shallow urethral groove bordered by urethral folds, the inner genital folds. Labial or scrotal swellings, the outer genital folds, border the urethral folds laterally. When the embryo has reached a length of 15 mm., the urethral groove is perceptibly longer in the male. In both sexes the tubercle elongates to form a phallus, the termination of which enlarges as the glans penis or the glans x;v^<gjjSiF' A B Fig. 287.—Development of the male external genitalia of man. A, indifferent stage from which either sex may develop; B, early, and C, later stages; a, anus;/, genital folds; g, urethral groove; p, genital tubercle; r, genital ridge (outer genital folds); s, scrotum. (From Kingsley's "Comparative Anatomy of Vertebrates," after O. Hertwig.) In the male the urethral folds unite along the median line to form an enclosed tubular urethra, the line of fusion persisting as the raphe of the adult penis. Closure takes place last in the region of the glans and is completed during the fourth month. An external raphe and an internal septum persist along the line of union of the scrotal swellings. At the end of the phallus the prepuce or foreskin, which encloses the glans, is formed by an ingrowth of epidermis around the glans. By the degeneration of the central cells of this ingrowth, the prepuce is separated from the glans except on its anal side where the glans and the prepuce remain connected by the frenulum of the prepuce. In the female changes occur more slowly. Instead of
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