Studies on the effects of some genetic lethal factors on the embryonic development ofDrosophila melanogaster : VI. An analysis of the mutantS9.

1. The development of the embryonic lethal mutantS9 was studied by means of serial sections and by observations on living embryos. 2. Lethality does not occur in the embryonic stage in all cases; a variable proportion of individuals of the lethal genotype dies in the pupal or larval stages. 3. The primary abnormality in the affected embryos is a disturbance of the distribution of the cleavage nuclei, which causes the blastoderm to be fragile at its anterior end, and consequently to lead to deformations in the later development of this region. 4. In the later stages of development, morphogenetic movements and cellular differentiation are disturbed in ways which differ greatly in individual cases, so that a wide variety of abnormalities is found in late embryos.