64 FROM FISH TO PHILOSOPHER 



bules are almost equally impermeable to urea and main- 

 tain a high concentration in the urine as against a low 

 concentration in the blood, although they are simultane- 

 ously carrying on a variety of chemical exchanges be- 

 tween these two fluids. But these two cellular structures 

 —the respiratory epithehum of the elasmobranchs and 

 the renal tubules of the mammals— are the only physi- 

 ological membranes known that are virtually impermea- 

 ble to this substance. 



The second step in the evolution of the urea-retention 

 habitus was to recover the urea from the glomerular fil- 

 trate by tubular reabsorption, as glucose and other valu- 

 able substances had been reabsorbed by the earlier verte- 

 brates. How the elasmobranchs do this is also a mystery; 

 the reabsorptive process is an active one specifically in- 

 volving urea molecules, and it is the only instance of 

 active reabsorption of urea known. It appears that this 

 reabsorptive operation is carried out not by a unique seg- 

 ment but by tubule cells which in other animals do not 

 have this function. Whatever the mechanism of re- 

 absorption, it saves some 90 per cent of the filtered urea 

 from excretion. 



The elasmobranch kidney maintains such a nice bal- 

 ance between the filtration of urea and its conservation 

 by the renal tubules that, with allowance for a small 

 and unavoidable loss of urea through the gills, the os- 

 motic pressure of the blood is maintained at a level that 

 will draw just sufficient water from sea water to meet 

 the animars needs. The mechanism appears to work with 

 great simplicity: as the water absorbed through the gills 

 dilutes the blood, the urine flow increases, increasing the 

 excretion of urea and thereby reducing the blood urea 

 concentration, which in turn reduces the absorption of 

 water through the gills, reduces the urine flow, and starts 

 a new cycle of urea retention, so that the animal is al- 

 ways supphed with enough free water to meet its urinary 

 requirements. 



