230 LECTURE XI. 



Schmiedeberg l is of the same mind, only he believes that the urea is 

 produced from ammonium carbonate. We can imagine that the ammo- 

 nium carbamate is an intermediate product, as indicated by the following 

 formulae : 



X 0(NH 4 ) /0(NH 4 ) X NH 2 



C=0 -* C = - C = O -f H 2 O 



X O(NH 4 ) X NH 2 X NH 2 



Ammonium Ammonium Urea 



carbonate carbamate 



Consequently no important difference seems to exist between the last 

 two views. Hoppe-Seyler 2 and Salkowski 3 look upon the formation of 

 urea in another manner. They assume that cyanic acid and ammonia are 

 first produced from albumin: 



NH 2 



NCOH + NH 3 - CN . O . NH 4 -> 



Cyanic acid Ammonium cyanate 



F. Hofmeister 4 has finally proposed a third hypothesis. He assumes 

 that urea is produced by an oxidation synthesis. From this point of 

 view, a CONH 2 group is formed by the oxidation of albumin or its amino 

 acid, which, at the moment of its formation, unites with the NH 2 residue 

 of ammonia, when the latter is oxidized, thus producing urea. 



Of the three theories mentioned, that of Hoppe-Seyler and Salkowski 

 seems to us the least probable. It has the least experimental support. 

 Again, cyanic acid has not been discovered in the organism. The Nencki- 

 Schultze-Schmiedeberg and the Hofmeister hypotheses, on the other hand, 

 have many observations substantiating their claims. In the first place, 

 numerous experiments have established the fact that ammonium carbonate, 

 and all those other ammonium salts which are capable of being converted 

 into it in the tissues, are changed into urea by the animal organism. 5 

 This applies to the carnivora as well as to the herbivora. It had seemed, 

 it is true, as if the former were an exception. After the administration of 

 sal-ammoniac, NH 4 C1, to rabbits, the increased elimination of urea corre- 



Arch. exp. Path. Pharm. 8, 1 (1879). 



Physiologische Chemie, Berlin, 1881, pp. 809 and 810. 



Z. physiol. Chem. 1, 1 and 374 (1877). 



Arch. exp. Path. Pharm. 33, 198 (1894); 37, 426 (1896). 



W. v. Knieriem: Z. Biol. 10, 263 (1874). E. Salkowski: Z. physiol. Chem. 1, 1 

 (1877). Cf. also L. Feder: Z. Biol. 13, 256 (1877). I. Munk: Z. physiol. Chem. 2, 29 

 (1878-79). E. Hallervorden: Arch. exp. Path. Pharm. 10, 124 (1879). F. Walter: ibid. 

 7, 148 (1877). Corander: ibid. 12, 76 (1880). J. Pohl and E. Miinzer: ibid. 43, 28 

 (1900). 



