THE URINARY ORGANS. 



273 



concave depressions into which the rounded ends of 

 the second layer or pear-shaped cells are adjusted. 

 Two or more layers of irregular interstitial cells inter- 

 vene between the apices of the pear-shaped cells. 

 The interstitial cells divide regularly by karyokinesis 

 and are then crowded to the surface to replace the 

 superficial cells that normally exfoliate. There are 

 no glands in the bladder, but solid cell projections 

 are sometimes found that resemble glands. The blad- 

 der is a part of the allantois, a vesicular evagination 

 of the hind-gut. The bladder epithelium, therefore, 



Pavement cell. 



Pear-shaped 

 cell. 



Pavement cells. 



Interstitial cells. 



Fig. 206. Epithelial cells from the bladder. 



is of hypodermic origin, while that of the ureter is 

 from the mesoderm. 



A vascular submucosa intervenes between the mu- 

 cosa and the muscularis. This is a thin layer of 

 areolar tissue, but sufficient to give the mucosa 

 apparent elasticity and enable it to move upon the 

 muscularis. 



The muscular coat consists of smooth muscle fibers 

 which may be divided into bundles of outer longi- 

 tudinal fibers, a middle strong circular layer, and an 

 imperfect inner longitudinal or diagonal stratum. 



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