SECT. 9] PROTEIN METABOLISM 1137 



for of the three in actual use it is much the most wasteful. Ammonia 

 is clearly the most efficient end product, for it involves no wastage 

 of carbon, but of the other two the carbon/nitrogen ratio is i /2 in 

 urea and i/o-g in uric acid. In other words, two atoms of nitrogen 

 can be got rid of at the expense of only one carbon atom in urea, 

 but only 0-9 in uric acid. These amounts may not be individually 

 considerable, butcollectively they may make all the difference between 

 an efficient and an inefficient species. Ackermann has compiled an 

 interesting table showing the nitrogen-removing efficiencies of many 

 urinary constituents, and has emphasised this point. Moreover, it is 

 obvious that uric acid excretion involves the wastage of a great deal 

 more chemical energy than urea, thus: 



Heat of combustion 



(cals. per gm. mol.) 

 Ammonia 906 



Urea 152-6 



Uric acid 462-1 



Uric acid, then, as the main end product of protein metabolism, may 

 be said to be more ingenious than the other two, but less efficient. 

 The proposition that the circumstances in which the embryonic 

 life has to be passed ultimately govern the form in which the nitrogen 

 is excreted is thus not so far-fetched as it sounds. During the last 

 fifty years much attention has been paid to the comparative study 

 of nitrogen excretion, but the methods of the older workers, such as 

 Krukenberg and Griffiths, were so unreliable that the earlier litera- 

 ture may be neglected. More recently, the researches of Przylecki; 

 Delaunay and others have begun the erection of a solid structure 

 of knowledge about the forms in which nitrogen is excreted. We are 

 thus acquiring, as it were, a wide series of phylogenetic base-lines 

 on which ontogenetic phenomena can be superimposed. The general 

 conclusions of these workers support the idea of an association between 

 aquatic life and the excretion of ammonia and urea, on the one hand, 

 and between terrestrial life and the excretion of uric acid, on the other 

 hand. But the important point is that the life of the embryo is the 

 key, not the life of the adult. An animal may live all its life in the 

 sea, but if its eggs are laid and develop on land it may be predicted 

 that its main nitrogenous end product will be uric acid. Mammals, 

 from the chemico-embryological viewpoint, count as aquatic animals , 

 since the excretion of nitrogenous waste products through the placenta 

 is analogous to their excretion into water. 



