110 THE ANATOMY OF VEETEBRATED ANIMALS. 



Mammalia, where tlie bladder, the genital ducts, and the 

 ureters, all open separately from the rectum into a genito- 

 urinary sinus. 



In the male sex, as development advances, this genito- 

 urinary sinus becomes elongated, muscular, and suiTounded, 

 where the bladder passes into it, by a peculiar gland, the 

 lirostate. It thus becomes converted into what are termed the 

 fundus, and neck of the bladder, with t)ie prostatic and mem- 

 branous portions of the urethra. Concomitantly with these 

 changes, a process of the ventral wall of the cloaca makes 

 its appearance, and is the rudiment of the intromittent 

 organ, or penis. Peculiai- erectile vascular tissue, developed 

 within this body, gives rise to the median corpus spongiosum 

 and the lateral corpora cavernosa. The penis gradually pro- 

 trudes from the cloaca ; and while the corpus spongiosum 

 terminates the anterior end of it, as the glans, the corpora 

 cavernosa attach themselves, posteriorly, to the ischia. The 

 under, or posterior, surface of the penis is, at first, simply 

 grooved ; by degrees the two sides of the groove unite, and 

 form a complete tube embraced by the corpus spongiosum. 

 The penial urethra is the result. 



Into the posterior part of this penial urethra, which is fre- 

 quently dUated into the so-called bulbus urethrw, glands, 

 called Cowper's glands, commonly pour their secretion ; and 

 the penial, membranous, and prostatic portions of the 

 urethra (genito-urinary sinus) uniting into one tube, the 

 male definitive urethra is finally formed. 



In sundi-y birds and reptUes, the penis remains in the 

 condition of a process of the ventral wall of the cloaca, 

 grooved on one face. In ornithodelphous mammals the 

 penial m-ethra is complete, but open behind, and distinct 

 from the genito-ui-inary sinus. In the Didelphia the penial 

 lu-ethra and genito-urinary sinus are united into one tube, 

 but the corpora cavernosa are not directly attached to the 

 ischium. 



Certain Reptilia possess a pair of eversible copulatory 

 organs situated in integumentary sacs, one on each side of 

 the cloaca, but it does not appear in what manner tljese 



