272 THE PHYSIOLOGY OF REPRODUCTION 



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 observa- 

 tions, 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. 



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. 1 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 intimately bound up 

 with the development of the embryo. Exposure to cold, which 

 delays development, also diminishes the excretion of carbonic 

 acid. 2 Experiments on the eggs of cold-blooded animals 3 show 

 that those conditions which favour development, such as high 

 temperature, also lead to an increase in the C0 2 excretion. 



The same problem has been attacked in a different way by 

 Tangl. 4 He determined, by means of a calorimeter, the heat 

 produced by the combustion of eggs at different stages of their 

 development. There is a gradual diminution of the caloric 

 value as development goes on, indicating that chemical energy 

 is used up in the process of development. In the case 

 of the chick the difference between the caloric value of the 

 fresh egg and that of the developed chicken is 16 calories. 

 These 16 calories represent the chemical energy which has been 

 used up for what Tangl calls the " work of development." 



But since Bohr's work has shown that the chemical 

 energy which disappears during development is completely 

 transformed into heat, it would be better to replace the 



1 Bohr and Hasselbalch, " Uber die Kohlensaureproduktion des Huhner- 

 embryos," Skandinav. Arch. f. Physiologic, vol. x., 1900. 



2 Pembrey, " On the Response of the Chick, before and after Hatching, 

 to Changes in External Temperature," Journal of Physiology, vol. xxvii., 

 1894. 



3 Bohr, " Uber den respiratorischen Stoffwechsel beim Embryo kaltbliitiger 

 Tiere," Skandinav. Arch. f. Physiologic, vol. xv., 1904. 



4 Tangl, " Beitrage zur Energetik der Ontogenese : I. Die Entwicklungs- 

 arbeit im Vogelei," Pfluger's Archiv, vol. xciii., 1903 ; vol. cxi., 1908. 



