ORGANIC EVOLUTION 



205 



duct, into which they later find admittance. The tubules 

 (fig. 131c) increase in number and differentiate internally. 





Fig. 131. Diagram of the development of amphibian gill slits 

 gill arches, pronephros and kidney, a, is a horizontal(frontal) 

 section of the body of a young larva, showing the relations 

 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; ihe 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 



