UROGENITAL SYSTEM. 



345 



folds which develop into the labia of the genital opening in the female 

 while in the male they furnish a part of the scrotal envelope. 



The most primitive type of the cloacal penis is found in the chel- 

 onians (fig. 343) and crocodiles, and slightly more developed in the 



FIG. 344. -Ventral cloacal wall and penis of Rhea (schematized), after Boas, b, 

 blind sac;/, corpus fibrosum; g, seminal groove; g', its continuation along blind sac; o, 

 opening of blind sac. Mucous membrane dotted, seminal groove black. 



ostriches and some of the aquatic birds. In these the ventral or 

 anterior wall of the cloaca and its lip become specialized by the develop- 

 ment in it of a longitudinal band of fibrous tissue, covered on the 

 cloacal side by cavernous tissue (containing large spaces, which on 



F IG - 345- Diagrams of male urogenitalia in 7, monotreme; II, marsupials; and ///; 

 monodelphs, after Weber, a, anus; b, bladder; c, cloaca; cc, corpus cavernosus urethra, 

 cp, corp. cav. penis; cd, Cowper's gland; p, perimeum; pg, prostate gland; r, rectum; , 

 symphysispubis; t, testis; u, ureter; v, vasdeferens; vg, vesicular gland; vm, ventral muscles. 



being filled with blood render the whole firm and enlarged erectile 

 tissue). The cavernous tissue is marked by a longitudinal groove 

 through which the seminal fluid from the urogenital sinus runs. Be- 

 sides the enlargement caused by the filling of the cavernous tissue with 



