Abstract SEVERAL investigators have shown that the hatchability of hens’ eggs is a hereditary character which is dependent on factors that not only are of vital importance to the embryo’s own life but also determine the physical characteristics of the egg. Data concerning physical characteristics of the egg that affect hatchability have been submitted by a few investigators but their observations are not entirely in agreement. Dunn (1922) reported an investigation on the relationship between the weight and hatching quality of White Leghorn eggs. He found a significant superiority in hatchability of medium-sized eggs over large ones, but reported that there was no correlation between the mean weight of the eggs laid by each bird and their hatchability. Jull and Haynes (1925) in a study involving eggs from 24 Barred Plymouth Rocks, found a tendency for hatchability to decrease as the size of egg increased, but their numbers were not large . . .