Abstract THE genetic aspects of hatchability and the knowledge concerning them have been discussed recently by Jull (1932). It has been established that cross-breeding often increases hatchability and that inbreeding often decreases it. These results of cross-breeding and inbreeding have been attributed chiefly to the different degrees of heterozygosity likely to be produced by these types of matings. Three characters lethal to the embryo when present in homozygous condition have been reported by Dunn (1923), Landauer and Dunn (1930), and Byerly and Jull (1932), respectively. Physical characters of the egg, such as weight, shape, and color, have little relation to hatchability within the normal range of variation of these factors. Evidence presented herein indicates that cross-breeding improves hatchability in inverse proportion to the hatchability of the parental stocks; that further improvement in hatchability is likely to be obtained by mating F1 females to males of an unrelated breed; that genetic factors . . .