THE UROGENITAL SYSTEM 



319 



Minor calyces arise by the continued subdivision and branching of the 

 major calyces. Further branching to the twelfth generation produces the 

 collecting tubules and the medullary portion of the kidney. Those of 

 the fifth generation to the number of twenty to eighty for each renal calyx 

 become the papillary ducts which convey urine from the collecting tubules 

 and which open directly into each of the renal calyces. (Fig. 283) 



Fig. 284. — Schematic diagrams to show the relations of pronephros, mesonephros, 

 and metanephros at various stages of development. In the adult male the mesonephric 

 (Wolffian) duct is retained as the ductus deferens. (From Patten's " Embryology of the 

 Chick.") 



Each of the collecting tubules terminates in a slight swelling or vesicu- 

 lated enlargement, upon which a mass of nephrogenic tissue rests as a cap. 

 From this mass arise excretory tubules, which subsequently acquire 

 connexion with the collecting tubule. (Fig. 283) Differentiation of the 

 excretory tubules begins with the formation of vesicular cell-clusters which 

 separate from the remaining nephrogenic tissue. These vesicles elongate 

 into excretory tubules and their ends enlarge to form Bowman's capsules. 



