NQRMAL PROCESSES OF ENERGY METABOLISM 617 



casions a more active production of carbon dioxid per unit of mass than 

 takes place in adult tissues. This has been demonstrated by Farkas for the 

 eggs of the silkworm, by Bohr for the embryo snake, by Bohr and Hassel- 

 balch, and by llasselbalch alone for the developing chick, and by Bohr for 

 the embryo guinea pig. That this greater evolution of carbon dioxid is the 

 expression of a greater liberation of energy also is rendered perfectly cer- 

 tain by the calorimetric measurements made by Farkas of the heat of com- 

 bustion of unincubated arid incubated silkworm eggs and those of Tangl 

 on the eggs of the cadaver fly ; by similar measurements made by Tangl and 

 by Tangl and Mituch on unincubated and incubated hen's eggs; and by 

 the direct calorimetric determinations of the heat produced in the develop- 

 ing hen's egg made by Bohr and llasselbalch. 



Bohr and Hasselbalch found on the fifth day of incubation of the hen's 

 egg a production of CO 2 amounting to 2000 c.c. per kilogram of embryo 

 per hour as against 718 c.c. per kilogram and hour for the adult hen (Reg- 

 nault and Reiset). The CO 2 production from the eighth to the twenty- 

 first day (end) of incubation was only a little greater in the embryo 

 than in the adult hen, but was sufficiently high for the authors to feel justi- 

 fied in concluding that it was "a condition for the organization of the new 

 tissue and not alone for the maintenance of tissues already formed." Grafe, 

 in reviewing this work, lays special emphasis on the fact that the highest 

 energy production takes place at a time when the work of differentiation 

 is most active. Bohr had previously supported this view with the evidence 

 derived from his study of embryo snakes. Increasing the temperature 

 from 15 C. to 27 C. increased the CO 2 output of an embryo weighing 

 3.8 gm. about 2.8 times, while the same increase in temperature raised 

 the output of an embryo weighing 0.5 gin. exactly four times. The greater 

 increase produced in the younger embryo, Bohr believes, was due to the 

 greater change in the intensity of the developmental processes. That is, the 

 processes of new formation (differentiation) are more active in the younger 

 stage and it is this part of the developmental process which is responsible 

 for the more active metabolism. 



Tangl's results on the hen's egg indicate an average heat production 

 for the entire incubation period of 100 calories per kilogram per day as 

 against 71 calories per kilogram per day (at 1S-20) for the adult hen 

 found by E. Voit an increase of 41.3 per cent. Tangl concludes that the 

 energy required for development (Entwickelungsarbeit) is considerably 

 greater than that required for mere maintenance of the adult organism 

 (Erhaltungsarbeit). The difference he designates as Bildungsarbeit. 

 Bohr's findings on the pregnant guinea pig are not so convincing. The 

 average production of CO 2 in the embryo he found to be 509 c.c. per 

 kilogram and hour; that of the mother 462 c.c. per kilogram and hour - 

 an increase of only 10 per cent. Granted that the conditions of heat loss 

 were the same in the two, which is doubtful, the amount of metabolism 



