BIOCHEMISTRY OF THE SEXUAL ORGANS 285 



of cold-blooded animals^ show that those conditions which favour 

 development, such as high temperature, also lead to an increase in 

 the CO2 excretion. But the relation between oxygen consumption 

 and development is not a simple one. For as J. Loeb has shown 

 in the parthenogenetic development of unfertilised eggs the oxygen 

 consumption increases before the actual development begins. And 

 Warburg ^ found that one can prevent the segmentation of the egg 

 without diminishing the respiration. 



The problem of the- energy exchange during developmeiit has 

 been attacked in a different way by Tangl.^ 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 diminu- 

 tion of the "caloric value as development goes on, indicating that 

 chemical energy is used up in the process of development. For the 

 chick- the following "balance-sheet" can be drawn up from the 

 observations of Tangl and his collaborators. The caloric value of an 

 egg of average weight (54 gm.) is 87 Cals. The difference between 

 the caloric value of the fresh egg and that of the incubated egg is 

 23 Cals. These 23 Cals. represent the chemical energy which has 

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

 Calculated for 1 gm. of chick this amounts to 805 small calories. 



, Before Incubation. 



Caloric value of egg 87 Cals. . 



Total 



87 Cals. 



After Incubation. 



Transformed into heat • 23 Cals. 

 Present in body of developed 



cMck - . 38 „ 



Eemaining unused 26 „ 



Total 



87 Cals. 



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 term " work of development " by the 

 term " energy of development." 



The nature of the substances which by their oxidation furnish the 

 " energy of development " is different in the differeiit classes of 

 animals. In birds it is furnished, as we have seen, by the oxidation 

 of fats, and possibly also of the fatty group of the phosphorised fats. 

 In Mammals, in which development proceeds in utero and there is a 

 constant exchange of material between the mother and the foetus, the 

 investigation of these problems is more difficult, owing to the 

 complexity of the conditions. Investigations on the respiratory 



1 Bohr, " Tiber den respiratorischen Stoffweohsel beim Embryo kaltbliitiger 

 Tiere," SkandKn. Arch. f. Physiol., vol. xv., 1904. 



^ Warburg, " Untersuchungen iiber die Oxydationsprocesse in Zellen,'' 

 Miinchener Mediz. Wochenschr., 1912, p. 2550. 



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

 arbeit im Vogelei," PJlUger's Arch., vol. xoiii., 1903 ; vol. cxxi., 1908. 



