SECT. 9] 



PROTEIN METABOLISM 



1089 



We can now examine again the part played by ammonia, urea 

 and uric acid as excretory products. Another way of expressing 

 the relationship leads to the result shown in Fig. 326. Here, 

 assuming that the ammonia, urea and uric acid together make up 

 the total excreted nitrogen (which is not quite true), the partition 

 between them has been calculated as milligrams excreted by the 

 embryo each day in per cent, of the total nitrogen excreted by the 

 embryo each day. Between the 4th and 5th days, the uric acid only 

 accounts for 9 per cent, of the total nitrogen, but so rapidly does the 

 change occur that, between the 8th and 9th days, it accounts for as 



• c t " a > 

 ■ o (Oi 



Fig. 326. 



much as 90-7 per cent. It has therefore practically reached its adult 

 level, as is shown by the comparative standards to the right of the 

 graph. The shaded parts at the bottom represent urea and the 

 rest uric acid; they are figures taken from Meyer; von Knierem; 

 Schimansky; Meissner, and Steudel & Kriwuscha. 



It is clear that in the first week of development the relationship 

 of urea and ammonia to uric acid is altogether different from what 

 holds in the adult, but that in the last two weeks of development the 

 adult value is rigidly adhered to. These results throw light on the 

 fact that the allantoic fluid in the chick passes from pH 7-2 to 

 pH 6-0 in the last half of incubation, whereas, before the 9th day, it 

 has been constant at about pH 7-2 (see Fig. 211). 



