Studies on the effects of some genetic lethal factors on the embryonic development ofDrosophila melanogaster : III. An analysis op the mutantX 27.

1. The development of the embryonic lethal mutantX 27 was studied by means of serial sections and by observations on living embryos. 2. The embryos reaches a late stage of development, and survives after the time at which normal embryos hatch. 3. The earliest observable effect of the factor is upon the first stage of gastrulation, in which the formation of the ventral furrow is disturbed. 4. Some abnormalities of the final embryo, especially certain deformations of the hind-gut and the nervous system, originate in mechanical disturbances arising from the abnormalities in gastrulation. 5. Other abnormalities do not originate in this way, but are due to defective cell differentiation: the gut remains sac-like, the ectoderm remains unsegmented, and the musculature of the body wall is undeveloped. 6. It is probable that the failure of the musculature of the body wall to develop follows from the absence of segmentation in the hypoderm, and that a type of inductive mechanism is involved.