SECT. I] PHYSICO-CHEMICAL SYSTEM 239 



It is certain that there are constant differences between the eggs of 

 different breeds, but as a whole these are quite outweighed by 

 individual differences, and only appear when extended statistical 

 studies are undertaken. The eggs of other birds, however, do not fall 

 within these limits. Langworthy, for example, has shown that, in 

 the duck's egg, the shell may account for as much as 14 per cent, 

 of the whole weight. A similar result was found for the turkey and 

 the goose, while the guinea-fowl's egg has a shell of nearly 1 7 per 

 cent, of the whole weight. The wide series of Friese, shown in Table i, 

 seems to indicate that the larger the egg the more shell it has to 

 have : thus the canary's egg weighing just under 2 gm. has 4 per cent, 

 while the goose's egg which weighs 137 gm. has 14 per cent. Heinroth, 

 and Groebbels & Mobert, among others, have collected a great 

 many data of this kind for all varieties of bird, but their papers must 

 be referred to for the figures. Thus the fertilised embryo starts its 

 development on the surface of a mass of food only slightly diluted 

 with water, and surrounded by a further and much wetter supply. 

 This is reflected well by the work of Bellini, who found that the yolk 

 of the hen's egg was seven times as viscous as the white at the begin- 

 ning of development. (Alb. 3-4 units, yolk 28-5 units.) 



A good deal of work has been done on the variability of the 

 weights of the parts of the egg within a given species of fowl. Thus 

 Jull found that egg weight is the least variable factor, albumen 

 weight slightly more variable than egg weight, yolk weight con- 

 siderably more variable than albumen weight, and shell weight the 

 most variable. It would seem, therefore, as if a compensatory process 

 takes place during egg-production, the largest yolks having the 

 smallest whites, since the weights of the entire eggs do not vary as 

 much as the weights of the components. On the other hand, the 

 smaller eggs contain the highest percentage of albumen and shell 

 and the lowest percentage of yolk. Jull also studied closely the 

 seasonal variations, which may be quite considerable, finding that 

 the component parts of the egg contribute in different degrees at 

 different times of the year towards the total egg weight. The question 

 as to which part of the egg is mainly responsible for large or small 

 eggs is still debated, for Curtis concluded from his observations that 

 it is the egg-white, while Atwood found many indications contrary 

 to this. Statistical studies on the egg of the tern have been made by 

 Rowan, Parker & Bell; Rowan, Wolff, Sulman, Pearson, Isaacs, 



