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 
Fic. 344.—Ventral cloacal wall and penis of Rhea (schematized), after Boas. 5, 
blind sac; f, corpus fibrosum; g, seminal groove; g’, its continuation along blind sac; 0, 
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 beconie 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 
Fic. 345.—Diagrams of male urogenitalia in J, monotreme; JJ, marsupials; and,JIT; 
monodelphs, after Weber. a, anus; 0, bladder; c, cloaca; cc, corpus cavernosus urethra, 
cp, corp. cav. penis; cd, Cowper’s gland; p, perinzeum; pg, prostate gland; 7, rectum; s, 
symphysis pubis; ¢, testis; u, ureter; v, vas deferens; 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 
