242 THE UNFERTILISED EGG AS A [pt. iii 



Elderton & Tildesley; and by Watson, Watson, Pearson, Karn, 

 Irwin & Pearson. 



The division of birds into the two classes of nidicolous (those 

 which hatch as squabs) and nidifugous (those which hatch as downy, 

 feathered and active chicks) has been shown to extend to the com- 

 position of their eggs by several investigators. Davy found that the 

 eggs of the nidicolous birds had thinner and more fragile shells, 

 which took up a less proportion of the weight of the whole egg than 

 the shells of nidifugous birds. Thus the wren's egg-shell weighs only 

 5 per cent, of the whole egg weight, while the hen's weighs lo per 

 cent. Da\y's figures show very clearly that the main reservoir of 

 solid is the yolk and the main reservoir of water is the egg-white. 



Tarchanov carried the matter further, and observed that the yolks 

 of nidicolous birds always formed a smaller proportion of the total 

 amount of material inside the egg than in nidifugous birds. Thus, 

 for the former class the egg-white accounts for about 78 per cent, 

 of the egg and the yolk for 22 per cent., while, in the latter class, 

 the egg-white accounts for about 55 per cent, and the yolk for 45. 

 All these differences are probably related to the shorter incubation- 

 time of the nidicolous eggs; and, as will be seen later, there are not 

 wanting indications that the yolk of these is less tightly packed with 

 food-material and more rich in phosphatides. 



1*4. The Chemical Constitution of the Avian Egg as a whole 



The composition of the egg as a whole is further considered in 

 Table 2, where it is noticeable that the analyses of water and ash 

 have not been significantly improved upon between 1863 (the date 

 of the first analysis, Payen's) and the present time. The later figures 

 for protein and fat are, however, much the more reliable. It should 

 be observed that there is an approximately equal quantity of fat and 

 protein at the disposal of the embryo, though the former is, of course, 

 in the yolk, and the latter is preponderantly in the egg-white. This 

 protein-fat equality is by no means the rule in all eggs, and, as we 

 shall see later, the eggs of fishes depart widely from it. There appear 

 to be only small differences between the eggs of different kinds of 

 birds in protein content. At one time it was thought that the duck's 

 egg was particularly rich in fat, on the authority of Commaille's 

 analyses, but Liihrig has since then brought it into line with all the 

 others. It does seem, however, to have a considerably higher per- 



