Fig. 117. The development of 

 the pronephros, mesonephros, and 

 metanephros. Left: The mesodermal 

 somites from 5 to 35. From 5 to 16, 

 the pronephros consists of solid 

 tubules and a solid duct. From 20 to 

 2 5 is an active region of differ- 

 entiation of the mesonephros. Since 

 the individual parts of the meso- 

 nephros begin to form in anterior 

 regions first, the fully formed renal 

 capsule is shown opposite somite 21. 

 Opposite somite 26 a mesonephro- 

 genic vesicle is shown which formed 

 from the nephrotome. At the level 

 of somite 25 the mesonephric vesi- 

 cle has changed to an S-shaped tube. 

 At somite 23 the mesonephric tu- 

 bule begins to develop a renal cap- 

 sule. At somite 21 the capsule is 

 completely formed and blood vessels 

 connecting with dorsal aorta have 

 formed within it. The network of 

 capillaries in the renal capsule is 

 called a glomerulus. The opposite 

 end of the mesonephric tubule con- 

 nects with the mesonephric duct 

 and the excretory products follow 

 the path indicated by arrows to the 

 allantois. Development of a renal 

 capsule of the metanephros is shown 

 from somites 33 to 30. Note that a 

 new excretory duct develops which 

 begins as an outgrowth of the meso- 

 nephric duct and runs forward. 

 This is the ureter. The metanephric 

 tubule connects with the ureter. 



SOMITES 

 5l 



10 



15 



20 



25 



30 



35 



S 



H 



ES 



SOLID 

 ■PRONEPHRIC TUBULE 



SOLID 

 -PRONEPHRIC DUCT 



HOLLOW 

 -MESONEPHRIC DUCT 



E3 



-n&: 



s 



S 



lS 



RENAL CAPSULE 

 MESONEPHRIC TUBUL6 



GLOMERULUS 

 DEVELOPMENT OF CAPSULE 



-CONVERSION TO S SHAPE 

 ■MESONEPHROCENIC VESICLE 

 -NEPHROGENIC TISSUE 



METANEPHRIC TUBULE 



-METANEPHRIC VESICLE 

 ALLANTOIS 



197 



