124 FROM FISH TO PHILOSOPHER 



foreshadowed among some of the Pernisylvania Am- 

 phibia and persisting primitively in the warts of living 

 toads. But waterproofing of the body did not wholly 

 solve the problem because, with neither gills nor skin to 

 participate in the process, the kidney now became the 

 sole means of regulating the composition of the body 

 fluids and the water requirement for urine formation set 

 the Hmits to the animal's freedom and independence of 

 environment. Now, more than ever before, survival 

 hinged on the efficiency of the kidney in maintaining the 

 water balance of the body. 



This crisis was met by a profound transformation in 

 the function of the reptihan kidney, supplemented by a 

 change in protein metabolism. It has been noted that 

 the major waste product requiring excretion by the kid- 

 ney is the nonvolatile end-product of the metabolism of 

 protein nitrogen. In the fishes and Amphibia this had 

 been urea, and so it continued to be in the 'amniotic' 

 stem that led to the mammals. The reptiles, however, 

 faced with a need of water conservation no less pressing 

 than that encountered by the marine elasmobranchs and 

 teleosts, overhauled their method of protein metabolism 

 and replaced urea by uric acid, which, molecule for 

 molecule, carries twice as much nitrogen out of the body, 

 and thus osmotically requires only half as much water 

 for the same quantity of protein metaboHzed. But more 

 important is the fact that uric acid is relatively insoluble 

 in water, and yet it readily forms a highly supersatu- 

 rated solution from which it separates as a fine, amor- 

 phous precipitate or as microscopic crystals. Once out 

 of solution, it exerts no osmotic pressure. The reptiles 

 (and birds) take the utmost advantage of this unique 

 property; they deposit uric acid by tubular excretion (in 

 addition to filtration) in the tubular urine in very high 

 concentrations— the ureteral urine of the chicken, for ex- 

 ample, may contain the extraordinary concentration of 

 21 per cent, or some three thousand times the concen- 

 tration in the blood— and as this urine passes to the clo- 



