266 THE PHYSIOLOGY OF REPRODUCTION 



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

 of iron which is almost completely absent in 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-stufE. 



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 out more clearly the difference 

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



100 parts of tlie Contain — 



Ash of KjO. NajO. CaO. MgO. FejOj. PjOj. SiOa- CI. 



White of egg 31-41 31-57 2-78 2-79 0-57 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, slight 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 investi- 

 gations 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 varjang from 0.0129 per cent. FejOg to 

 00086 per cent. FcgOg, the maximum found being 0-0167 per 

 cent. FejOj. (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 Umited, 

 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. FejOs, so 

 that the seasonal diminution which normally appears is prevented. 



^ Albu and Neuberg, Physiologic und Pathologic des Mineralstoffwechaels 

 Berlin, 1906, p. 241. 



' Hartung, "Der Eisengehalt des Hiihnereies," Zeitachrift fiir Biologic, 

 vol. xliii., 1902. 



