. Chemical embryology. Embryology. 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 lim
. Chemical embryology. Embryology. 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. 1171). 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. 1312). But Kaieda has brought forward convincing evidence 00 90 Points © 80 -| ⢠Averages /^ J \o 70 ±SI &/ e e\ GO --C^^ Q 50 CPta ;1J /© \ i i/» \ 40 7 0 ^ 30 -â a / >-^ 20 E 10 E I 1 1 1 , , 1 , , , :,,,,! 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 16th 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). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Needham, Joseph, 190
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