Studies on the effects of some genetic lethal factors on the embryonic development ofDrosophila melanogaster : I. A preliminary survey of some sex-linked lethal stocks, and an analysis of the mutantLff 11.

1. Out of seventy sex-linked lethal mutants examined, twelve were found to cause death in the embryonic stage. 2. In six of the embryonic lethals, no visible abnormalities occurred, but the embryos failed to hatch. 3. In the other six cases there were obvious morphogenetic disturbances. Detailed developmental studies were made of these, of which the mutantLff 11 is reported in the present paper. 4. The embryo of the mutantLff 11 reaches a late stage of development, and lives beyond the normal time of hatching, but it is abnormal in many respects. Differentiation of the tissues is relatively normal, but the organ systems are extremely disorganized. 5. The disorganization of the late embryo arises from abnormalities in gastrulation, which have two major effects:- 1) the invagination of the posterior mid-gut rudiment and the germband posteriorly; 2) the formation of a constriction behind the head region. 6. It seems likely that these abnormalities arise from disturbances in the relative timing of certain developmental events, in particular in the precocious formation of the embryonic membrane material. 7. The abnormalities in gastrulation are discussed with respect to the light they throw upon the mechanism of gastrulation in normal embryos, and especially upon the significance of the cephalic furrow. 8. Certain resemblances between the mutant development and that of coleopteran embryos are pointed out, and their evolutionary significance is discussed.