DIFFERENTIATION OF THE NEPHROGENIC TISSUE 



203 



larger collecting ducts open. The nephrogenic tissue forms the cortex 

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

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

 depressions. The human fetal kidney is thus distinctly lobated, the loba- 

 tions persisting until after birth, a condition which is permanent in rep- 

 tiles, 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 col- 

 lecting tubules run parallel and converge to the papilla?. 



4 5 



FIG. 212. Semidiagrammatic figures of the anlage and differentiation of renal vesicles 

 and early developmental stages of uriniferous tubules of mammals. I and 2, Anlage and suc- 

 cessive 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 glo- 

 merulus beginning with a tubular anlage showing a well-developed S-shape (Huber). 



Differentiation of the Nephrogenic Tissue. In stages from 13 to 19 

 mm., the nephrogenic tissue about the ends of the collecting tubules 

 condenses into spherical masses that lie in the angles between the buds of 

 new collecting tubules and their parent stems (Fig. 212). One such 

 metanephric sphere is formed for each, new tubule. The spheres are con- 



