SECT, i] PHYSICO-CHEMICAL SYSTEM 251 



fat percentage (Cross), yolk-protein percentage (Cross), egg-protein 

 percentage (Cross), egg-phosphorus percentage (Cross), chick- 

 phosphorus percentage (Cross). 



It appears, however, that the constitution of the egg-proteins may- 

 be influenced by the presence of unusual proteins in the diet of the 

 hen, and that this may influence hatchability. Pollard & Carr have 

 reported the results of feeding the following proteins to laying hens : 

 wheat, rye, corn, oats; kaffir, barley, peas, soya, hemp; buckwheat, 

 popcorn, sunflower seed. 



The first group of four (all, of course, being fed alone) were very 

 efficient for the production of normal eggs; the second group (of 

 five) permitted the hens to lay eggs but the eggs were hardly hatch- 

 able at all, while the third group allowed of no eggs. Pollard & 

 Carr studied the egg-proteins in all cases and obtained evidence of 

 tryptophane deficiency in the second group, so that they concluded 

 that a minimum tryptophane content was essential for successful 

 development through hatching. It is unfortunate that their results 

 were never published in full. 



The effect of sex on the chemical composition of the egg has been 

 discussed by Riddle. As is well known, in some, probably most, 

 animals, the male produces two kinds of spermatozoa which are not 

 equal in their prospective sex value, i.e. some which will give rise 

 to females and some which will give rise to males. In birds, on the 

 other hand, the dimorphism of the germs exists not in the spermatozoa 

 but in the egg-cells. The female produces two kinds of eggs of unequal 

 prospective sex value. Riddle found that pure wild species of doves 

 and pigeons were ideal material for studies on sex, since very abnormal 

 sex-ratios could easily be obtained from them, and his studies led 

 him to the view that sex was more a matter of metabolic level or 

 rate of protoplasmic activity than anything else. But what concerns 

 us here are the consistent differences which he was able to demon- 

 strate between male and female eggs. 



Pigeons generically crossed, when not permitted to lay many eggs, 

 produce only males, but when made to lay many eggs produce first only 

 males, and eventually "under stress of overwork" only females. These 

 facts and their proper conditions having been ascertained previously 

 by extensive statistical investigations, the way lay open for the 

 chemical analysis of the two sorts of pigeon's eggs. 900 analyses 

 were made and more than 12,000 yolks weighed. 



