BIOCHEMISTRY OF THE SEXUAL ORGANS 271 



whicli is due to luteins. Feeding with grains produces a Hglit 

 yellow yolk, a dark yellow yolk results if grass and herbs are 

 given, while feeding with worms leads to the production of an 

 even darker reddish yolk. What the changes are in the colour- 

 ing matter of the yolk has not yet been ascertained.^ 



During the development of the chick a considerable portion 

 of the fat disappears. In other words, a certain amount of 

 chemical energy, which in the fresh egg is present in the form of 

 fat, disappears. Liebermann ^ has shown, for instance, that of 5'4 

 grm. of fat present in a fresh egg only 2'7 grm. can be recovered 

 when the chick is hatched. The fate of the chemical energy 

 which has thus disappeared has been accounted for completely 

 by the observations of Bohr and Hasselbalch,^ which are the 

 most exact and comprehensive investigations on the subject 

 of the metabohsm of the embryo. 



They showed that the respiratory quotient of the developing 

 egg — that is, the ratio of the amount of COg excreted to the 

 amount of Og absorbed — is 0'71. Such a quotient indicates the 

 oxidation of fat. From the amount of COg 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 de- 

 velopment 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 Cal. 



The amount of heat actually given oflF . . . 12-16 Cal. 



This remarkable agreement in so comphcated an experiment 

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



' For the morphological distribution of the constituents of the yolk, see 

 Waldeyer, " Die Geschlechtszellen," in Hertwig's Handbuch der Entwicklungs- 

 lehre der Wirbeltiere, vol. i., Jena, 1903. 



* Liebermann, " Embryochemische Untersuohungen," Pflilger's Archiv, 

 vol. xliii., 1888. 



- Bohr and Hasselbalch, " Uber die Warmeproduktion und den Stoff wechsel 

 des Embryo," Skandinavieches Arch. f. Physiologic, vol. xiv., 1903. 



