250 THE UNFERTILISED EGG AS A [pt. iii 



Stewart & Atwood reported that chicks hatched from pullet eggs 

 were neither so large nor so vigorous as those hatched from the eggs 

 of hens two or three years old. Whether there is here a direct effect 

 on the chick of the age of the hen, or whether the effect is indirect, 

 due to the small size of the egg, may be well questioned. 



What relations exist between the chemical constitution of the egg 

 and the percentage "hatchability" are at present obscure, owing 

 perhaps to the comparative crudity of our estimation methods. The 

 work of Pearl & Surface indicated definitely that differences in the 

 hatchability of eggs are determined by or associated with innate 

 differences in the individual hens which laid them, that these dif- 

 ferences are probably inherited, and that variations within rather 

 wide limits in certain environmental factors, e.g. the temperature, 

 during incubation, are of secondary importance in determining the 

 death or the hatching of the embryo. Hatchability of embryos would 

 appear then to be, like fecundity, a heritable character. The experi- 

 ments of Lamson & Card confirmed the conclusions of Pearl & 

 Surface, but although some physico-chemical mechanism is un- 

 doubtedly at work, these statistical studies gave no hint as to its 

 nature. 



Dunn determined to probe further into it. In his first paper he 

 argued that if hatchability was associated with constitutional vigour, 

 it should show a correlation with such a value as the chick mortality 

 in the first three weeks of post-natal life. Experimentally this was 

 not the case, e.g. post-natal mortality remained the same, although 

 in two instances the pre-natal mortality was on the one hand ex- 

 tremely high (20-39 per cent, hatchability) and on the other hand 

 extremely low (80-100 per cent, hatchability). It therefore seemed 

 likely that mortality before and after hatching is determined by quite 

 different factors. The more specific influences operating in embryonic 

 life must doubtless be looked for in the physico-chemical constitution 

 of the unincubated egg. 



Hays & Sumbardo, in a search for such influences, were able to 

 exclude statistically fresh weight, length, diameter, specific gravity, 

 shell thickness, outer and inner shell-membrane thickness, porosity 

 and imbibition of water from 25 per cent, salt solution. Other factors 

 which have been excluded are percentage of protein in the diet of 

 the laying hen (Rosedale), percentage of yolk-pigment (Benjamin), 

 evaporation rate of the egg (Dunn), yolk-fat percentage (Cross), egg- 



