THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 401 



mids. These are apparent on the surface of the kidney and constitute the 

 surface lobulation, but are not clearly defined in the interior. 



The formation of renal papilla? (p. 396) corresponds to the formation of 

 pyramids only to a certain point, for some of the tertiary pyramids appear only 

 near the surface and consequently do not have corresponding papilla. This 

 accounts for the fact that frequently the number of pyramids apparent on the 

 surface does not correspond with the number of papilla?. The surface lobula- 

 tion is very plainly marked in kidneys up to and for a short time after birth. It 

 then disappears and the surface becomes smooth. At the same time the con- 

 nective (mesenchymal) tissue of the renal columns is largely replaced by the 



Secondary 

 renal 

 column Secondary 



renal 



pyramid c i 



secondary 



Fig. 359. — Frontal section of the kidney of a human fcetus of 19 weeks (17.5 cm.). Hauch. 



epithelial elements of the gland so that in the adult kidney the columns are not 

 clearly defined. 



The capsule of the kidney is derived from the mesenchyme which surrounds 

 the anlage of the organ (Fig. 353). This mesenchyme is transformed into fibrous 

 connective tissue and a small amount of smooth muscle, forming a layer which 

 closely invests the kidney and dips into the hilus where it surrounds the blood 

 vessels and the end of the ureter. The connective tissue and muscle of the 

 ureter are also derived from the mesenchyme. 



Cortex and Medulla. — As the convoluted renal tubules develop in the 

 metanephric blastema (p. 396), they form a cap-like mass around the group of 



