SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. II9 



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 efifect 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 stage of development or differ- 

 entiation zvhich determines the size of yolk is not accurately 

 measured by the body zveight. It is of course w^ell 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 zueight zve 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 

 Qgg shape in a domestic fowl. In the case of yolk zueight, 

 hozuever, 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 &gg seems to be most closely related to the age of 

 the bird and least affected by the other natural causes of vari- 

 ation in Qgg 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. 



