THE UROGENITAL SYSTEM 



435 



become the papillary ducts, which convey urine from the collecting tubules 

 and which open directly into each of the renal calyces. (Fig. 362) 



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. 

 ,^$^g*^? FIRST CERVICAL 



V- PRONEPHROS 



--FIRST THORACIC 

 -GLOMERULUS 



NEPHROGENIC, 

 TISSUE 



METANEPHROS 

 PRIMITIVE DUCT/ 



CLOACA 



NTESTINE 



-^^^'^'^---FIP 



URETER 

 RST LUMBAR 



Fig. 360. — A diagram of pro-, meso-, and metanephroi in a 5-6 mm. human embryo. 

 Somites are cross-hatched. (Redrawn after Braus.) 



From this mass arise excretory tubules, which subsequently acquire 

 connexion with the collecting tubule. 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. 

 Subsequent changes involve the attachment of the tubule to the adjacent 

 collecting tubule, the elongation of the excretory tubule, and the ingrowth 



