202 



THE UROGENITAL SYSTEM 



the pelvis. The apices of the pyramids are termed renal papilloR and through 

 them the larger collecting ducts open. The nephrogenic tissue forms the cortex 

 of the kidney, and each subdivision of it, covering the tubules of a pyfamid 

 peripherally, is marked off on the surface of the organ by grooves or depressions. 

 The human fetal kidney is thus distinctly lobated, the lobations persisting until 

 after birth, a condition which is permanent in reptiles, birds, and some mammals 

 (whale, bear, ox). The primary pyramids are subdivided into several secondary 

 and tertiary pyramids. Between the pyramids the cortex of nephrogenic tissue dips 

 down to the pelvis, forming the renal columns (of Bertin) . The collecting tubules, 

 on the other hand, extend out into the cortex as the cortical rays -or pars radiata 

 of the cortex. In these rays, and in the medulla of the kidney, the collecting 

 tubules run parallel and converge to the papillae. 



Fig. 212.— Semidiagrammatic figures of the anlage and differentiation of renal vesicles and early 

 developmental stages of uriniferous tubules of mammals. 1 and 2, Anlage and successive stages in the 

 differentiation of renal vesicles, as seen in sagittal sections; 3, section and outer form of tubular anlage 

 before union with collecting tubule at the beginning of S-shaped stage; 4 and 5, successive stages in the 

 development of the tubules, Bowman's capsule, and glomerulus beginning with a tubular anlage showing 

 a well-developed S shape (Huber). 



i 



DIFFERENTIATION OF THE NEPHROGENIC TISSUE 1 



In stages from 13 to 19 mm., the nephrogenic tissue about the ends of the 



collecting tubules condenses into spherical masses which lie in the angles between 



