SECT. 9] 



PROTEIN METABOLISM 



1077 



is very intense after the 4th day and before the 9th day. At later periods, 

 although excretion of urea is 

 still going on, it only just suc- 

 ceeds in keeping abreast of 

 the wet weight. It was at once 

 noteworthy that this intensive 

 period of urea production oc- 

 curred exactly between the 

 carbohydrate period and the 

 period associated with the pre- 

 dominance of fat metabolism. 

 The effect may, of course, be 

 due to other causes than to a 

 specially intense combustion of 

 protein during this period. For example, it may be due to a limiting 

 factor such as the small size of the Hver, or rather to the incapacity of 

 the embryonic Uver at this stage to turn urea into uric acid. That the 

 developing liver can act in this way is probable from what we know 

 of the desaturation process in 

 embryonic metabolism (see p. 

 1 1 7 1 ) . If this were the case, how- 

 ever, an inflection in the curve 

 of milligrams per cent, should 

 appear at the time when the 

 liver takes on this function. The 

 activities of the enzyme arginase 

 may also be involved (see p. 

 13 1 2). But Kaieda has brought 

 forward convincing evidence 



Days 5 



Fig. 314. 



that the urea which the chick embryo produces is due to the deamina- 

 tion of protein breakdown-products. He injected each amino-acid 

 into hen eggs before incubation and estimated the urea formed by 

 the 1 6th day, with the following average results: 



Control, Allantois 

 ,, Embryo 



After injection, both 

 Average increase 



mgni. 

 I -00 

 1-37 

 2-37 

 3 "So 

 1-23 (ranging from 0-4 to 207 



according to the amino-acid) 



