BIOCHEMISTRY OF THE SEXUAL ORGANS 277 



coloured by pigments, which are probably allied to the bile 

 pigments.^ There are variations in the strength and thickness of the 

 shells of different eggs. Sometimes they are, exceptionally thin and 

 soft. Riddle^ holds that there is a loose association between the 

 production of inadequate shells and the early death of the embryo 

 before hatching. Feeding with calcium does not prevent the 

 production of inadequate shells, so that lack of calcium cannot be 

 its cause. 



The shell membrane consists of a substance belonging to the group 

 of the keratins. It is very rich in sulphur (about four per cent. S), 

 and, on hydrolysis, yields a relatively large amount of cystin (see 

 p. 288). It also loses weight during development. The loss consists 

 of organic substances and amounts to 0'2 gm. 



The chief constituents of the white and the yolk of the egg 

 are water, proteins, fats, and phosphorised fats, while carbohy- 

 drates as such are almost entirely absent. The white of the 

 egg contains on the average 0*47 per cent, dextrose, the yolk 

 0'14 per cent., while for the whole egg the average is G'45 per cent, 

 dextrose.^ 



The proportion in which these constituents are present in the 

 white and in the yolk of the egg differs, as will be seen from 

 the following table giving the total composition of both these 

 parts. The figures for the yolk are taken mainly from Riddle's 

 estimations.* 



Another important difference in the composition of the white and 

 the yolk of the egg is tp be found in the relative quantities of the 



' Krukenberg, "Farbstoife der Vogeleiersohalen," Verhandlimgeii d. Phys. 

 Med. Geselhchaft, Wiirzburg, vol. xvii., 1883. 



^ Biddle, "Studies on the Physiology of Reproduction in Birds," Amei: 

 Jour, of Physiol., vol. Ivii., 1921. 



' Hepburn and St. John, "Dextrose content of the Hen's Egg," Prot: 

 Amer. Soc. Biol. Chemists, 1921, Jour. Biol. C'/iem., vol. xlvi. 



* Riddle, " Studies on the Physiology of Reproduction in Birds," Amer. .hmr. 

 of Physiol., vol. xli., 1916. This paper contains a full bibliography. 



