SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 119 
curve. This curve which represents analytically the law of the 
increase in yolk weight with the age of the bird is of the same 
type as the curves which have been found to fit various kinds 
of growth data in both animals and plants. 
The first interpretation that suggests itself is that this in- 
crease in the size of the yolk is a direct effect of the analogous 
increase in the size of the individual. That the body weight as 
well as yolk weight normally continues to increase at least to 
the end of the second year was seen by comparing the body 
weights at the beginning, middle and end of the observations. 
However, the curve showing the increase in yolk weight is not 
parallel to a curve which shows the increase in body weight at 
the same period. During the period of most rapid increase in 
yolk weight the increase in body weight was too small to be 
certainly distinguished from the fluctuations due to temporary 
variations in the amount of food and waste present in the 
body. This indicates that the stooge of development or differ- 
entiation which determines the size of yolk is not accurately 
measured by the body weight . It is of course well known that 
the different organ systems of the body show different growth 
stages at the same time and that the reproductive system shows 
most rapid growth not long before the beginning of its func- 
tional activity. 
In yolk weight we find a logarithmic approach of successively 
formed structures to a type. Pearl* (’07) found this true of 
leaf number per whorl in Ceratophyllum and later (’09) t of 
egg shape in a domestic fowl. In the case of yolk weight , 
however, it is certain that this is not due to the continued pro- 
duction of the like parts but to the condition of the individual 
at the time the part is produced. 
The change in yolk weight due to the age or maturity of 
the bird has been discussed at length because the weight of this 
part of the egg seems to be most closely related to the age of 
the bird and least affected by the other natural causes of vari- 
ation in egg size. The weight of albumen and shell also both 
* Pearl, R., Pepper, O. M. and Hagle, F. H. Variation and Differen- 
tiation in Ceratophyllum, Carnegie Institution Publ. No. 58, pp. 1-136. 
1907. 
t Pearl, R. Regulations in the Morphogenetic Activity of the Ovi- 
duct. Jour. 'Exp. Zool., Vol. VI, 1909, pp. 339-358. 
