1136 PROTEIN METABOLISM [pt. m 



liquid-tight boxes, and never afterwards forgot. Even the eggs of 

 water birds, which might be supposed to have the opportunity of 

 excreting substances into the water around them, have impenetrable 

 fat-impregnated shells, as Loisel has shown. The highest avian groups, 

 exemplifying as they do the most complicated form of nitrogen 

 excretion, would thus represent the crowning achievements of the 

 uricotelic line of evolution. And the fact that, between the 2nd and 

 5th days in the chick's development, it excretes ammonia and urea 

 with no uric acid would thus be a recapitulation of its pre-terrestrial 

 or aquatic ancestry, entirely analogous with its gill-clefts. Moreover, 

 the coincidence is exact, for it is just between the 2nd and 5th days 

 that the embryo manifests its morphologically piscine characteristics. 

 Fig. 323 showed the milligrams of ammonia, urea and uric acid 

 present in embryo, amniotic and allantoic liquid throughout in- 

 cubation, expressed in terms of 100 gm. dry weight of embryo; 

 in other words, it showed what 100 gm. dry weight of embryo has 

 manufactured in the way of nitrogenous end products by any given 

 time. Table 138 showed the relations between these substances in 

 another way. Although ammonia, urea and uric acid are excreted 

 by the chick embryo during its development, the two first-named 

 molecules only account for an insignificant part of the total nitrogen 

 excreted. There is a progression in ontogeny from the smallest to 

 the largest molecule, and from the most to the least efficient excretory 

 product. It could be argued, of course, that, if the chick can excrete 

 urea early in its development, it ought to be able to return to this 

 practice after hatching; but this would be to neglect one of the 

 most characteristic features of embryonic life, namely, its continual 

 tendency to lose pluripotence, and to move towards a stable, 

 "crystalline", or set state. The chick embryo begins to excrete uric 

 acid about the 6th day. Before that time, as we know from the work 

 of Przylecki & Rogalski, it possesses uricase, but after that time it 

 does not. Perhaps, then, when the reptiles came ashore they found 

 it was as well to leave their uricase behind them. As for avian 

 development, hatching is followed by a sudden burst of protein 

 catabolism, as we have seen in Section 6- 11, and this is probably 

 associated with the chick's enlarged opportunities for getting rid of 

 nitrogenous waste. 



There remains the consideration that no animal would excrete uric 

 acid as its main nitrogenous end product unless it was driven to it, 



