THE UKINIFEEOUS TUBULES 351 



mids. They are lined by tall columnar cells with an exception- 

 ally clear cytoplasm which has an affinity for the basic in prefer- 

 ence to the acid class of dyes. The nuclei are spheroidal or ovoid 

 in shape and lie in the basal portion of the cell. At their termi- 

 nation several ducts of Bellini frequently open into a common de- 

 pression or foveola which is lined by an involution of the layer of 

 transitional epithelium, derived from that of the renal calyx, by 

 which the free papillary portion of the Malpighian pyramid is 

 clothed. 



It is obvious that the entire uriniferous tubule, from the capsule 

 of Bowman to the duct of Bellini, is a continuous tube whose epi- 

 thelial wall, supported by a thin homogeneous basement mem- 

 brane varies in character in each succeeding portion. Thus the 

 proximal and distal convoluted portions and the ascending limbs 

 of Henle's loops possess a granular, rodded or striated, acidophile, 

 secreting epithelium ; the capsule of Bowman has thin cells of an 

 endothelial type ; the descending limb and loop of Henle are 

 lined by flattened finely granular and faintly acidophile epithe- 

 lium; the curved and straight collecting tubules and ducts of 

 Bellini possess a clear columnar epithelium. It should also be 

 noticed that the several portions of the uriniferous tubule occur 

 in different topographical subdivisions of the kidney and that, 

 therefore, each of the subdivisions contains only certain character- 

 istic portions of the uriniferous tubule. Thus are found in the 



T Malpighian body. 



Neck of the tubule. 

 CORTICAL LABYRINTH. <j Proximal convoluted portion. 



Distal convoluted portion. 

 [^ Arched collecting portion. 



i Spiral portion of the convoluted tubule. 

 Ascending limb of Henle's tubule. 

 Straight collecting portion. 



( Descending limb of Henle's tubule. 

 BOUNDARY ZONE OF \ ->.-> * TT i > .L t, i 



THE MEDULLA 1 Ascending limb of Henle's tubule. 



1UJS IVliiDULLiA I CM i L IT j.- ' 



( Straight collecting portion. 



f Descending limb of Henle's tubule. 

 PAPILLARY ZONE OF J Loop of Henle's tubule. 



THE MEDULLA ] Straight collecting portion. 



[ Duct of Bellini. 



