278 THE PHYSIOLOGY OF REPRODUCTION 



inorganic constitnents as they are present in the dry residue,^ both as 

 inorganic salts and in organic combination. 



100 parts of Dry 



Residue of 



White of egg 



Yolk of egg - 0-27 0-17 038 006 0024 1-9Q Q-35 



It will be seen that the yolk is distinguished by the presence of 

 iron which is almost completely absent from the white, and by its 

 richness in phosphorus. Although the percentage of iron present in 

 the yolk is very small, it is nevertheless greater than in almost any 

 other animal or vegetable food-stuff. 



As a rule the proportions in which the inorganic elements are 

 present are given in terms of percentages of the ash. Such a table,^ 

 which perhaps brings otit more clearly the difference between the 

 white and the yolk of the egg, may be given here : — 



100 parts of the Contain" — 



Ash of KjO. NagO. CaO. MgO. FeAr PA- SiOg. CI. 



White of egg - 31-14 31-57 2-78 2-79 ■ OSf 4-71 1-06 28-82 



Yolk of egg 9-29 5-87 13-04 2-13 1-65 65-46 0-86 1-95 ' 



There are, of course, sUght variations between different eggs in 

 the amount of mineral constituents present in the ash. It is possible 

 that there are such variations even in the eggs laid by one .and the 

 same bird at different periods. Systematic investigations on this 

 point have been made only with reference to the iron. These 

 observations show that more iron is present in eggs laid in spring 

 than in eggs laid by the same bird in autumn, the amounts varying 

 from 0'0129 per cent. FcgOg to 0'0086 per cent. ^6263, the maximum 

 found being 0'0167 per cent. Fe^Og. (The percentage is calculated 

 for the dried yolk.) This fact probably explains the very exaggerated 

 statements which have been made concerning the production of eggs 

 rich in iron by keeping hens on a diet rich in iron. The careful 

 observations of Hartung ^ show that there is indeed a distinct effect 

 produced by such a diet, provided that it is given over a prolonged 

 period — two months or more. But the effect of such a diet is limited, 

 and does not go beyond the physiological maximum. The percentage 

 of iron present in eggs laid under these conditions remains fairly 

 constant, and is about equal to the maximum found under normal 

 conditions, namely 0'0165 per cent. Fe^Og, so that the seasonal 

 diminution which normally appears is prevented. 



1 Bunge, " Der Kalk und Eisengehalt unserer Nahrung," Zeitsch. f. Biologie, 

 vol. xlv., 1904, p. 532. 



2 Albu and 'Sevibeitg, Physiologie imd Pathologie des Mineralstoffweohsels, 

 Berlin, 1906, p. 241. r 



3 Hartung, "Der Eisengehalt des Hiilinereies," Zeitsch. f. Biologie, vol. xliii., 

 1902. 



