THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



395 



column it turns and grows cranially between the latter and the mesonephros. 

 The stalk (or ureter) elongates accordingly (Fig. 350). 



About the fifth week, four evaginations from the primitive renal pelvis appear 

 one cephalic, one caudal and two central (Figs. 350 and 352). These may be 

 considered as straight renal tubules of the first order. The distal end of each 

 then enlarges to form a sort of ampulla, and from each ampulla two other 

 evaginations develop, forming tubules of the second order. From the ampulla 

 of each secondary tubule two tertiary tubules grow out; and this process con- 



Mesonephros 



Mesonephric dui 



Junction of meson, 

 duct and ureter 



Cephalic evagination 



_) Metanephric blasterr.a 



Central evaginations 



-, Caudal evagination 



FIG. 350. From a reconstruction of the anlage of the kidney, etc., of a human 

 embryo of 11.5 mm. Schreincr. 



tinues in a similar manner until twelve or thirteen divisions occur, the final 

 divisions occurring during the fifth month. The tubules grow into the mesen- 

 chyme which surrounds the pelvis and which forms the so-called metanephric 

 blastema, or nephro genie tissue (Fig. 351). 



If the straight tubules were to remain in this condition, only four would open 

 directly into the pelvis, corresponding with the four primary evaginations. In 

 the adult, however, many hundreds open into the pelvis; consequently extensive 

 changes of the early condition must take place. These changes are similar to 



