SECT. 9] 



PROTEIN METABOLISM 



065 



of development the percentage of cystine in the embryonic protein is 

 fairly steady, but during the 

 first and last days it rises. The 

 rise at the end of the period 

 might be due to the appearance 

 of feathers and the cystine of 

 their keratin, but, as Table 108 

 shows, their weight is small, 

 and will probably not account 

 for it. Cahn's explanation in- 

 volved the postulate of a central 

 fixed nucleus in the embryonic 

 protein molecule, of which 

 arginine would be a constituent 

 member, while cystine would 

 not. These are hypotheses on 

 which much further work might 

 be profitably carried on. 



According to Calvery, the 

 cystine/cysteine ratio of the 

 hen's egg falls from 5-9 to o-8 

 during development. 



Days -^ 



Fig. 300. 



Table 134. 



9-3. The Relations between Protein and Non-protein Nitrogen 



Many investigators, wishing to unravel the processes by which the 

 egg proteins are transformed into those of the embryonic body, ha\'e 

 estimated the non-protein and free amino-acid nitrogen in various 



68-2 



