ORGANIC EVOLUTION 



205 



duct, into which they later find admittance. The tubules 

 (fig. i^ic) increase in number and differentiate internally. 



a 



[. Diagram of the development of amphibian gill slits 

 ches, pronephros and kidney, a, is a h9rizontal (frontal) 

 n of the 'body of a young larva, showing the relations 



FIG. 131. 



section of 'the 'body of a young larva, snowing 

 of the newly formed pronephros to the dorsal aorta in front 

 and to the cloaca behind; h, is the hyo-mandibular gill cleft. 

 *, 2,3, 4, the succeeding four branchial clefts; b, is a dissec- 

 tion of an older larva, showing the branchial blood vessels, 

 and the true kidneys attaching to the pronephric duct; c, is a 

 diagram of a single nephridial tubule from the kidney. The 

 arrows at the left indicate the course of the capillary blood 

 circulation through the glomerulus; the arrow at the right 

 indicates the path of excreta to the ureter. 



Each acquires connection with one of the capillaries joining 

 the renal arter y with the post cava. All of one side become 

 combined together in a single organ, the kidney. The 



