58 Physiology of the Kidney 



of the embryo; a multitude of adaptations, most of which 

 concern the preservation of the internal environment, had to 

 be effected to liberate the organism from its original aquatic 

 heritage. One of the most important of these adaptations 

 consisted in a subtle change in the method of protein com- 

 bustion. Instead of degrading protein nitrogen to urea, as 

 had the fishes and Amphibia, the reptiles overhauled their 

 metabolic machinery and degraded their protein nitrogen to 

 uric acid. Uric acid is a very peculiar substance: it is almost 

 insoluble in water, and yet it readily forms highly supersat- 

 urated solutions; the reptiles secrete it in the tubular urine 

 as a concentrated, supersaturated solution, then as the tubular 

 urine passes to the cloaca the uric acid precipitates out, leav- 

 ing most of the water in the urine free to be reabsorbed into 

 the blood, while the uric acid itself is expelled as an almost 

 dry paste. This same uric acid adaptation, Hke so many other 

 reptilian characters, is found in the birds,^®'^^ for the birds are 

 but warm blooded reptiles with feathers and wings. 



When the teleosts faced dessication in the briny sea many 

 of them completely discarded their luxus glomeruli. In view 

 of the fact that in the arid-living reptiles and the marine 

 birds the need for water conservation is equally extreme one 

 might expect some of them to be aglomerular too, but no 

 aglomerular reptile or bird has thus far been described. The 

 reptilian-avian kidney is, however, headed in that direction, 

 for the once elaborate glomerular tuft is reduced to a few, 

 in some cases only two, capillary loops, and contains a great 

 amount of inert connective tissue. It is as though these ani- 

 mals, having found the glomeruli largely superfluous but 

 needing to flush the uric acid-rich secretion of the tubules 

 down to the cloaca, had stopped short of the complete oblit- 

 eration of the glomeruli and retained a vestige of the filtering 



