THE URINARY ORGANS. 



197 



glomerulus include those of the walls of the blood-vessels and of the 

 interstitial tissue, as well as those of the capsular epithelium. 



2. At the neck the flattened epithelium abruptly becomes cuboidal 

 and rapidly assumes the character of the lining of the convoluted 

 tubule. The existence of ciliated epithelium at the neck or within 

 the capsule in the mammalian kidney has been asserted, but not satis- 

 factorily established ; in many of the lower animals, however, as in 

 the amphibians, the presence of cilia is readily demonstrated, as is like- 

 wise the existence of tubules opening directly into the peritoneal cavity. 

 Such trumpet-shaped orifices the nephrostomata represent a 

 partial persistence of the primitive type of excretory organ, in which 

 the tubules pass directly from the body-cavity to the outer surface. 



3. The proximal convoluted tubule is clothed with low co- 

 lumnar or cuboidal cells, whose granularity and transparency vary 

 with the stage of secretion, as do likewise the thickness of the epi- 

 thelium and the size of the lumen of the canal. The outer zone of 



v 



Section of kidney of amphiuma : the peritoneal 

 surface (b, b) exhibits one of the nephrostomata 

 (<?), lined with ciliated cells; d, glomerulus sur- 

 rounded by capsule ; u, uriniferous tubules ; z>, 

 capillaries filled with red blood-cells. 



Portions of the constituents of the medulla 

 from the human kidney : A , B, collecting 

 tubules ; C, D, descending and ascending 

 limbs of Henle's loop ; E, blood-vessel. 



the epithelium, next the basement-membrane, presents more or less 

 clearly the vertical striation distinguishing rod-epithelium. The 

 demarcation of the individual cells is not sharply marked, their 

 boundaries being indistinctly defined. 



4. The epithelium of the spiral tubule closely resembles that of 

 the preceding portion, consisting of similar low columnar elements 

 possessing granular protoplasm but less marked striations. 



