206 



THE EXCRETORY SYSTEM 



and while it is taking place the urogenital sinus itself is subdividing 

 into a dorsal and ventral portion. The allantois and mesonephric 

 ducts connect with the dorsal portion of the urogenital sinus. The 

 ventral, so-called phallic portion, is later in\'olved in the develop- 

 ment of the urethra region of the genital ducts of both sexes, espe- 

 cially with the penis of the male, and continues to expand to become 

 a wide, muscular-walled sac, the bladder. At the anterior ventral end 

 of this enlargement is a stalk of tissue, the urachus of the embryo, 

 but in the adult this is the middle umbilical ligament tying the 

 bladder to the body wall. The ureters shift their position so as to 

 join the wall of the developing bladder, and as the latter grows 

 forward it carries the ureters with it, separating them from the 

 mesonephric ducts which are retained to become functional in con- 

 nection with the reproductive system of the male. 



Fig. 131. — Photograph of the urethra of a female mouse. The lumen is lined by 

 a thin stratified epithelium. In the submueosa are mixed serous and mueous 

 glands which extend into the underlying skeletal muscle. 



The mammalian bladder has four coats of tissue making up its 

 wall, namely, the mucosa, submucosa, muscularis, and fibrosa. 

 (Fig. l.'^O.) The mucosa, like the ureters, is lined with transitional 

 epithelium. The behavior of this type of epithelium in the dis- 

 tended and empty bladder has already been described in the con- 

 sideration of epithelia. In the empt>' bladder the mucosa is wrinkled 

 by folding of the underlying submucosa. In some bladders there 

 are simple gland-like })ockets in the epithelial wall. TIkmt being 



