THE UROGENITAL SYSTEM 



303 



seminal groove of the reptile has become a tubular canal surrounded by 

 erectile tissue, the corpus cavemosum urethrae. This canal is used in 

 monotremes for the passage of sperm only, while urine passes from the 

 bladder to the cloaca by way of a urinary canal. In addition to the 

 erectile tissue of the corpus cavemosum urethrae, paired masses of 

 erectile tissue, the corpora cavernosa penis, make their appearance in 

 the phallus of monotremes. The posterior free portion of the phallus 



Fig. 270. — Diagrams of male urogenitalia in I, monotreme; II, marsupials; and III. 

 monodelphs. a, anus; h, bladder; c, cloaca; cu, corpus cavemosum urethrae; cp, corp. 

 cav. penis; cd, Cowper's gland; p, perineum; pg, prostate gland; r, rectum; s, symphysis 

 pubis; t, testis; u, ureter; v, vas deferens; vg, vesicular gland; i/w, ventral muscles. 

 (From Kingsley's "Comparative Anatomy of Vertebrates," after Weber.) 



of monotremes becomes more elongated than in reptiles, 

 by an integumentary fold, the preputial sac, which 

 erection. (Fig. 270) 



In marsupials and placental mammals, with the 

 the cloaca, the phallus becomes more independent as 

 The urinary canal of monotremes disappears, and the 

 to the outside through the urogenital canal or urethra, 

 with an enlarged corpus cavemosum urethrae persists. 



and is surrounded 

 disappears during 



disappearance of 



an external penis. 



urine is conveyed 



The glans penis 



In the female, it 



