THE URETERS 



203 



cells, hut the mid-region along the kidney has a superficial layer 

 of columnar cells and a basal layer of polyhedral cells. Through the 

 main ])ortion of the ureter from the kidney posteriorly the superficial 

 cells, which may have smaller cells interspersed, rest upon one or 

 two layers of polyhedral cells. Immediately below the epithelium 

 is a thin region of connective tissue surrounded by a circular band 



Nephridial 

 tubules 



Mucosa 



Submucosa 

 Musculans 



Fig. 128. — Diagram of ureter of the water snake and adjoining uriniferous tubules. 



-^ — Capillary 



Adveniitia 



Submucosa 



Circidar 



muscle 



Tratisitional 



epithelium 



Longitudinal 



muscle 



— Mesothelium 



Fig. 129. — Diagram of mammalian ureter (dog). 



of smooth muscle. A thin layer of connecti\-e tissue forms the most 

 external coat. 



Metanephric Duct. — In reptiles, birds, and mammals where the 

 metanephros is the functional kidney, the ureter occurs as a new 

 structure developing from the basal portion of the old mesonephric 

 duct, as already noted. (Fig. 128.) 



