SECT. 9] PROTEIN METABOLISM 1133 



Grigaut, who found a dialysis coefficient of 93 for urea, but only 

 74 for sodium urate. The haemato-encephalic barrier, according to 

 them, allows urea to pass easily, but not uric acid. And the first 

 substance retained in the mammalian circulation, if the kidneys are 

 impaired, is uric acid. Shut up as it is in its closed box, the chick 

 embryo would evidently find uric acid by far the most convenient 

 excretory product, for the two former would tend to diffuse through- 

 out the egg, and to establish themselves in equal concentration in all 

 its constituent regions, instead of being packed into a small store. 

 As it happens, the work of Kamei provides a striking verification 

 of this viewpoint, for as we have seen, he showed that, in the amniotic 

 liquid of the chick, although the uric acid concentration never rises 

 above a certain very low level, the ammonia and the urea rise con- 

 tinuously throughout development. It is easy to guess, therefore, 

 what would happen if all the nitrogen excreted by the embryo were 

 in the form of urea. As an illustrative calculation we may take the 

 uric acid present in the allantois at the end of incubation as 100 mgm. 

 (data of Fiske & Boyden; Needham; Targonski and others) — i.e. 

 about 33 mgm. of uric acid nitrogen or 66 mgm. of urea. This, 

 distributed over an egg of contents approximately 40 gm., would be 

 165 mgm. per cent. The egg would be uraemic (in the strict, not 

 the clinical, sense of the word). The normal figure for the urea- 

 content of human and bovine blood is about 25 mgm. per cent., 

 and the highest figure on record obtained by ingesting solid urea is 

 just under 100 mgm. per cent. In severe renal obstruction or 

 nephritis, it rises above 100, and may reach 300 or 400, but 165 

 is undoubtedly of the pathological order of magnitude, and if the 

 avian embryo had to suffer from a constant headache and other 

 symptoms before hatching, natural selection would hardly have 

 preserved it for our entertainment. These consequences could be 

 avoided by the use of uric acid. 



Such considerations lead to the suggestion that the form of excre- 

 tion of nitrogen adopted by an animal depends principally on the 

 conditions under which its embryo has to live. There is good evidence 

 that the combustion of protein substances as a source of energy is 

 much more marked in aquatic than in terrestrial embryos (Needham). 

 Table 161, constructed from as much of the information as is trust- 

 worthy, shows the partition between the substances comprising the 

 total material catabolised. Thus only 5 or 6 per cent, of the total 



