SECT. 9] 



PROTEIN METABOLISM 



1087 



but not to anything like the same extent as it does when the former is 

 passing over into the latter. 



Rapkine has pointed out that this peak in protein catabolism pre- 

 cisely coincides with the time of development at which chick embryos 

 are most suitable for use in providing the plasma for tissue cultures. 

 Whether the growth-producing substance is a proteose, a peptone, 

 a dipeptide, or even an amino-acid, it does at any rate seem clear 

 that it is a nitrogenous substance of protein origin, and it is therefore 

 interesting to note that it seems to be at its greatest concentration 

 in the embryo just at the time when there is a maximum of protein 

 catabolism. Carrel gives the 7th to the loth day as the period most 

 suitable for taking embryos for this purpose, and the peak, as we 

 have seen, occurs at 8-5 days of 

 development. It is also very in- 

 teresting that Remotti's curve 

 for the activity of the yolk pro- 

 teases reaches its maximum on 

 the 9th day of development (see 

 Fig. 423 fl). This brings up the 

 whole question of how the yolk 

 and white proteins are trans- 

 formed into the tissue proteins 

 during development. It is pos- 

 sible that the former are not 



1200 

 ? 1100 



o 



^ 1000 



g 900 



55 



,700 

 600 

 500- 

 400 



,300 

 200 

 100 



PERIOD OF 



CARBOHYDRATE 



COMBUSTION 



PERIOD OF 



FAT 

 COMBUSTION 



Days -5 



Fig. 325- 



reduced to their constituent amino-acids entirely, but only to a pro- 

 teose or peptone stage. If this were so. Carrel's growth-promoting 

 substance might simply be one of the normal intermediate products. 

 There have not so far been any researches designed to test this 

 interesting possibility. 



From the data in Table 138, it will be seen that the total number 

 of milligrams of nitrogen excreted by the embryo during its develop- 

 ment (or, more accurately, transmuted into the nitrogenous waste 

 products, urea and uric acid) is 11 -oo. Sendju, as we have seen, 

 estimated the total amounts of various amino-acids in the hen's egg 

 during the course of its development, finding a very slight increase 

 in the histidine, and a constancy in the arginine, lysine, and mono- 

 amino-acids, while in the tryptophane and tryosine, on the other 

 hand, there was a considerable falling off. He correlated this with 

 the formation of pigments, and did not take into account losses by 



