378 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



This pit is not a cloaca, since it is ectodermal in origin, while the 

 cloaca is derived from the entodermal part of the digestive tract. 



Fig. 402. — Development of the male external genitalia of man, after O. Hertwig. 

 A, indifferent stage from which either sex may develop; 5, early, and C, later stages; 

 a,, anus;/, genital folds; g, urogenital groove; p, genital prominence; r, genital ridge; s, 

 scrotum. 



In the monotremes the penis (,fig. 403, 7) is still cloacal in posi- 

 tion, but there is an advance over the sauropsida in that the groove on 



Fig. 403. — Diagrams of male urogenitalia in /, monotreme; //, marsupials; and 

 ///; monodelphs, after Weber, a, anus; b, bladder; c, cloaca; cu, corpus cavernosus 

 urethra, cp, corp. cav. penis; cd, Cowper's gland; p, perinaeum; pg, prostrate gland; r, 

 rectum; s, symphysis pubis; t, testis; u, ureter; v, vas deferens; vg, vesicular gland; vm, 

 ventral muscles. 



the dorsal surface has been converted 'into a canal which carries both 

 urine and sperm. The whole structure can be protruded and re- 

 tracted into a sheath formed from the loose mucous membrane of the 



