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 clitoridis. 



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 closing as in 

 the male, the urethral groove remains permanently open as the vestibule, 

 and the urethral folds persist as the labia minora. The labial swellings 

 elongate and become in part the labia majora. The elongation of the 

 phallus characteristic of the male does not occur in the female. The glans 

 however persists as the clitoris. Consequently, in respect to external 

 genitals, the female resembles the undeveloped male. 



