THE METANEPHROS 



209 



constitute the collecting tubules which form the greater part of the medulla of 

 the adult kidney. 



When the four to six primary tubules develop, the nephrogenic cap about the 

 primitive pelvis is subdivided and its four to six parts cover the end of each pri- 



=€ranlcU pole 



Zubuie 



Crantat 

 pole tubule 



Caudal 

 pole tubule 



Ureter 



Tertiary coL- 

 UctittQ tubule 



Fig. 203. — Diagrams showing the development of the primitive pelvis, calyces and collecting tubules of 

 the metanephros (based on reconstructions by Schreiner and Felix). 



mary tubule. As new orders of tubules arise, each mass of 

 nephrogenic tissue increases in amount and is again sub- 

 divided until finally it forms a peripheral layer about the 

 ends of the branches tributary to a primary tubule. The 

 converging branches of such a tubular "tree" constitute a 

 primary renal unit, or pyramid, with its base at the periphery 

 of the kidney and its apex projecting into the pelvis. The 

 apices of the pyramids are termed renal papilla and through 

 them the larger collecting ducts open. The nephrogenic 

 tissue forms the cortex of the kidney, and each sub-division 

 of it, covering the tubules of a pyramid peripherally, is marked 

 off on the surface of the organ by grooves or depressions. The 

 fetal kidney is thus distinctly lobated, the lobations persist- 

 ing until after birth. 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 Bertini). The collect- 

 14 



Fig. 204.— The 

 pelvis, calyces, and 

 their branches and a 

 portion of the ure- 

 ter, from the meta- 

 nephros of a 16 mm. 

 embryo (Huber). 



