NITROGENOUS COMPOUNDS 133 



a certain extent at least, or act as protein sparers. 

 The problem of the utilization of ammonium salts 

 subjected to much experimentation in the past has 

 recently been revived through the work of Grafe and 

 Schlapfer who have asserted that ammonium salts, 

 urea, and even nitrates may serve as sources of nitro- 

 gen for the animal organism, and they regard the 

 utilization as indicated by nitrogen retention as a 

 process of amino acid synthesis. These results have 

 been assailed by others on the ground that the observed 

 retention of nitrogen as a result of feeding the above- 

 mentioned compounds may be explained in other ways 

 than as a proof of amino acid synthesis. "There are 

 several ways in which they may be assumed to behave 

 in the organism. In the first and foremost instance 

 they may serve as pabulum for the alimentary bac- 

 teria, which in turn are destroyed in large numbers 

 in the digestive tract and can furnish a yield of per- 

 fect protein synthesized from simple compounds like 

 urea and the salts of ammonia. It is generally ad- 

 mitted that in certain species like the herbivora, in 

 which bacterial processes have a free play in the 

 gastro-intestinal tube, the contribution of dead bac- 

 terial bodies to the intake is by no means negligible." 

 "The feeding of urea or ammonium salts might lead 

 to an apparent nutritive advantage by depressing or 

 inhibiting the usual breaking down of nitrogenous 

 compounds in metabolism. This would accord with 

 the belief that the products of cellular waste them- 

 selves tend to impede cellular metabolism. Now that 



