284 THE PHYSIOLOGY OF REPRODUCTION 



and comprehensive investigations on the subject of the metabolism 

 of the embryo. 



The developing egg takes in oxygen and gives oft' C0 2 . Bohr and 

 Hasselbalch showed that the respiratory quotient of the developing 

 egg that is, the ratio of the amount of CO., excreted to the amount 

 of O 2 absorbed is 071. Such a quotient indicates the oxidation of 

 fat. From the amount of C0 2 excreted during a given period it is 

 possible to calculate the amount of fat oxidised during that period. 

 Under ordinary conditions the oxidation of fat produces heat which 

 can be determined experimentally. By calculating from the amount 

 of fat oxidised during development the amount of heat which would 

 be generated under ordinary conditions, and by actually determining 

 at the same time the amount of heat given off by the developing egg, 

 Bohr and Hasselbalch found during a period of twelve days : 



The amount of heat calculated from the amount of fat oxidised - 12'11 Cals. 

 actually given off - . - 12 - 16 



This remarkable agreement in so complicated an experiment 

 which is a triumph of the experimental skill of the observers shows 

 clearly that fat is the almost exclusive source of the chemical energy 

 which is used up during development. Another very important 

 conclusion can be drawn from these observations, namely, that all the 

 chemical energy which disappears during development reappears in 

 the form of heat; none is transformed in an unknown way into 

 energy of a different kind, or transferred to the developing embryo. 



This observation, which is of fundamental importance in its bearing 

 on our conceptions of life and living matter, was confirmed by Tangl. 

 The converse was also experimentally tested by Rubner ] in the case 

 of yeast. When yeast-cells undergo autolysis no energy is liberated. 

 These experimentally established facts dispose in a final and 

 conclusive manner of all speculations concerning the existence of 

 unknown forms of energy in living cells. 



The intensity of the metabolic changes which take place during 

 development, and which can be expressed by the amount of C0 2 

 excreted, is very great. 2 Calculated for the same unit of weight of 

 the animal, it is as great in the embryo as it is in the adult animal, 

 and may even exceed it. This is the case not only in birds, but also 

 in Mammals. These changes are bound up with the development of 

 the embryo. Exposure to cold, which delays development, also 

 diminishes the excretion of carbonic acid. 3 Experiments on the eggs 



1 Rubner, "Die Umsetzungswarme bei der Alkoholgarung," Arch.f. Hygiene, 

 vol. xlix. 



2 Bohr and Hasselbalch, "Uber die Kohlensaureproduktion des Hiihner- 

 embryos," Skandin. Arch. f. J'lt<isi<>l., vol. x., 1900. 



3 Pembrey, " On. the Response of the Chick, before and after Hatching, to 

 Changes in External Temperature," Jovr. of PkysioL, vol. xxvii., 1 894. 



