998 



ENERGETICS AND ENERGY-SOURCES [pt. iii 



the uric acid curve. If the embryo had been burning protein because 

 carbohydrate was absent or not easily obtained, then the uric acid 

 curve should have been depressed after the injection of glucose, but 

 this was never the case. 



So far the order in which the three great types of biologically 

 important molecule are combusted for the Ea. during the develop- 

 ment of the embryo has alone been taken into consideration, and not 

 the relative proportions in which they are used for this purpose. In 

 the case of the chick, the following figures were obtained by Needham 

 in 1927: 



Total material Correction for 



disappearing during material disap- 

 the whole of in- pearing but not 

 cubation (mgm.) combusted (mgm.) Result 



Carbohydrate... 166 



Protein ... 68 



Fat 



Same in per 



cent, of the 



total material 



catabolised 



2171 



40 



o 



105 



126 



68 



2066 



2260 



5-57 

 3-02 

 91-4 



SIGNIFICANT 

 LIMITS 



This is simply a more accurate statement of a fact which has fre- 

 quently been mentioned before, namely, that the main source of Ea. 

 in the hen's e^g: is fat. Other 



, 1 , iN.irrTpnN INJECTION 



embryos, however, do not 

 utilise fat to the same extent, 

 and Table 126 summarises the 

 data which show this. It largely 

 explains itself. The first two 

 items are the only ones in 

 which our knowledge is com- 

 plete, though doubtless not 

 final. Attention may be directed 

 to the fact that the amount 

 of carbohydrate combusted in 

 per cent, of the total amount 

 of food-stuflT combusted is of the same order in the frog as in the chick. 

 But this is not the case for protein and for fat, for in the former case 

 7 1 per cent, is protein and 22 per cent, fat, while in the latter case 6 per 

 cent, is protein and 91 per cent. fat. Here then is a distinct difference 

 between the metabolism of the embryonic chick and the embryonic 

 irog. The problem is to decide upon what biological difference to fix 

 as the correlate of this biochemical difference. Most probably the 



