II32 PROTEIN METABOLISM [pt. m 



9-15. Uricotelic Metabolism and the Evolution of the Ter- 

 restrial Egg 



We have now examined all the existing knowledge about the pro- 

 tein metabolism in embryonic life, and it is time to turn to a few 

 general considerations. One of the questions always asked by students 

 in biochemistry is why some animals should excrete urea, some am- 

 monia, some uric acid. As far as I know, they have not so far been 

 accustomed to receive any reasonable reply, and the problem has 

 been set down as one of those arbitrary dispositions of fate which 

 make Elementary Classes despair of biochemistry, but I think an 

 answer can be given. 



Fiske & Boyden, in their memoir on the nitrogen metabolism of 

 the hen's egg, raised an interesting point when they calculated that 

 1 5 per cent, of all the water in the egg at the beginning is needed 

 to excrete the 5 mgm. odd of uric acid which are present in the 

 allantoic liquid by the 1 1 th day of development. From that time 

 onwards re-absorption of water vigorously proceeds, no doubt for the 

 reason that, without it, all the water in the residues and in the body 

 of the embryo would be required to get rid of the uric acid that is 

 to be formed. It is as if the water acted as an endless belt conveyer, 

 transferring uric acid from the cells of the embryo into the allantoic 

 liquid, and then returning to transfer more. The fowl is always good 

 at absorbing water from its excretions, for, as Wiener and Sharpe 

 have shown, the glomerular urine in the adult is quite liquid, and 

 the cloaca absorbs great quantities of water. All terrestrial animals 

 do this to some extent, if the views of Cushny about the function 

 of the mammalian kidney tubules are correct. But in the hen's egg 

 it is obvious how closely the process as a whole is bound up with the 

 properties of uric acid. "A substance as soluble and diffusible as urea ", 

 say Fiske & Boyden, "could not possibly replace it as an end-product 

 when the organism and its excretions are confined to a closed system, 

 the walls of which are only permeable to matter in the gaseous state." 



This is a very important consideration. There appear to be only 

 three substances which are available in the animal kingdom for 

 carrying away the nitrogenous waste resulting from protein break- 

 down — ammonia, urea and uric acid. The first two of these com- 

 pounds are very soluble and diffusible; uric acid is not. Quantitative 

 expression of this fact has been given by Chauffard, Brodin & 



