BIOCHEMISTRY OF THE SEXUAL ORGANS 265 



some species the shell is coloured by pigments, which are pro- 

 bably derivatives of the bile pigments.^ 



The shell membrane consists of a substance belonging to the 

 group of the keratins. It is very rich in sulphur (about 

 4 per cent. S.), and, on hydrolysis, yields a relatively large 

 amount of cystin (see p. 276). 



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

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

 hydrates as such are almost entirely absent. 



The proportion in which these constituents are present in 

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

 the following table giving the total composition of both these 

 parts. 



Another important difference in the composition of the 

 white and the yolk of the egg is to be found in the relative 

 quantities of the inorganic constituents as they are present in 

 the dry residue,^ both as in organic salts and inorganic com- 

 bination. 



* Krnkenberg, " Farbstoffe der Vogeleierschalen," Verhandlungen d. 

 Phys. Med. Gesellschafl, Wiirzburg, vol. xvii., 1883. 



' Bunge, " Der Kalk und Eisengehalt unserer Nahrung," Zeitschrift f. 

 Biologie, vol. xlv., 1904, p. 532. 



