FORMATION OF URINE 579 



decreased elimination, possibly as the result of its more intimate 

 association with the blood proteins retarding its filtration, as 

 on the theory of an increased absorption. 



At present it seems that no conclusive evidence of the 

 theory that the concentration of the urine depends upon 

 reabsorption has been adduced. Much of the evidence points 

 to an active secretion, and the facts which do not directly point 

 to this may be explained as well on the theory of secretion as 

 on that of reabsorption. A verdict of not proven must be given, 

 but since in the nephridial tubules of the annelid the epithelium 

 is secretory, the onus of proving that the changes in the con- 

 centration and reaction of the urine are due to absorption lies 

 upon the supporters of this theory. 



The secretion theory does not raise the difficulty of explaining 

 why a mechanism involving the filtration under pressure of such 

 an enormous quantity of water with the sole purpose of having 

 it again reabsorbed has been evolved, or of attempting to 

 say at v/hat stage of evolution the epithelium of the nephridial 

 tubules reversed their function. It has been suggested that, 

 when animals became terrestrial, the need of conserving water 

 arose and the tubules took upon themselves this function. But 

 the tubules of aquatic animals are as well developed as those of 

 terrestrial animals. 



Those who accept the reabsorption hypothesis claim that 

 while such substances as urea are eliminated as fully as possible, 

 other substances which are normally present in the blood in 

 appreciable amounts are reabsorbed to the extent of maintain- 

 ing that amount. The first set of substances they call "non- 

 thi'eshold substances," the second " threshold substances." 

 But the differentiation between these is just as readily 

 explained on a theory of secretion as on a theory of reab- 

 sorption. 



While the evidence at present forthcoming does not directly 

 point to the changes in the urine as it passes down the tubules 

 being due to reabsorption, it by no means excludes the 

 possibility that some reabsorption may take place. 



The exiraordinary differences in the structure of the 

 ■epithelium in the convoluted tubules on the one hand, and of 

 the looped tubules of Henle on the other, suggests the possibility 

 that different processes may be carried on in these parts, that 



