444 



COMPARATIVE ANATOMY 



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 



A B 



Fig. 371. — Development of the male external genitalia of man. A, indififerent stage 

 from which either sex may develop; B, early, and C, later stages; a, anus;/, genital folds; 

 g, urogenital groove; p, genital prominence; r, genital ridge (outer genital folds); s, 

 scrotum. (From Kingsley's " Comparative Anatomy of Vertebrates," after O. Hertwig.) 



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. 



PERITONEUM PERlTONaJM 



URETHRAL GROOVE 



URETHRAL GROOVE 



a c. 0. 



Fig. 372. — Diagram showing four stages in the development of bladder and rectum 

 in the human embryo. As a result of the backward growth of a septum the cloacal 

 cavity becomes divided into a more dorsal rectal cavity and a ventral urogenital sinus. 

 Since the cloaca is lined by endoderm the lining of the bladder is chiefly endodermal. 

 (Redrawn after Coming's "Human Embryology.") 



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. Some observers state that the 

 scrotal swellings subsequently migrate posteriorly and unite behind the 

 penis, but this is denied by others. An external raphe and an internal 

 septum persist along the line of union of the scrotal swellings. At the end 



