io86 PROTEIN METABOLISM [pt. hi 



Allantoin should therefore be estimated quantitatively in the 

 allantois during the first week of incubation. Przylecki & Rogalski's 

 findings with regard to the appearance of xanthine oxidase are in 

 accord with those of the American school described in Section 14-10. 

 The only information as to the way in which the avian uric acid 

 is formed in the egg is due to the researches of Tomita & Takahashi. 

 Believing that the following chain of reactions took place : 



CH3 COOH 



CHOH + 3O - H^O-^CHOH 



COOH COOH 



lactic acid from tartronic 



carbohydrate breakdown acid 



COOH NH— CO 



CHOHfCO<JJ&-^CO CHOH 

 I - I I 



COOH -- NH-d:q 



tartronic -^ dialuric 



^"^ breakdown ^<^>^ 

 dialuric acid +urea-> uric acid, 



they injected tartronic acid and urea into hen's eggs and succeeded in 

 obtaining an increased formation of uric acid by the embryo, as shown 

 in Fig. 324. But some doubt may be expressed concerning their 

 control of the range of variation in normal eggs and Clementi con- 

 siders that urea cannot be an intermediate step in the synthesis of 

 sauropsid uric acid. 



9'5. Protein Catabolism 



We must now return to the question of protein combustion in the 

 chick. Knowing the amounts of ammonia, urea and uric acid present 

 each day during development, it is simple to calculate the amount 

 of protein burned. This gives the graph shown in Fig. 325, in which 

 the milligrams of protein combusted by 100 gm. of dry weight of 

 embryo per day are plotted against the time in days. A marked 

 peak appears at 8-5 days' development, the significance of which is 

 sufficiently indicated by the labels summarising other evidence. This 

 has been discussed in Section 7-7. One may say that a given weight 

 of embryo combusts 6 or 7 times as much protein on the 8th day of 

 development as it does on the 4th or the 1 6th day. The combustion 

 of protein thus goes on during both the carbohydrate and fat periods, 



