Studies on the effects of some genetic lethal factors on the embryonic development ofDrosophila melanogaster : IV. An analysis op the mutantX 20.

1. The development of the embryonic lethal mutantX 20 was studied by means of serial sections and by observations on living embryos. 2. There is considerable variation in the expression of the lethality in the embryo. Four types of abnormal embryos are produced:I andII, in which there are certain abnormalities in various organ systems;III, in which there is an irregular blastoderm, and no development after gastrulation; andIV, in which cell proliferation occurs, but not differentiation. 3. Intypes I andII, the chief abnormality is in the differentiation of the ectoderm, but the disturbances in it are of contrary kinds in the two types. Intype I, a complete nervous system is produced, but the hypoderm is absent ventrally; whereas intype II, the nervous system is nearly or quite absent, and the hypoderm is complete. 4. The abnormal embryos oftype I are identical with some of those which previous workers have obtained by treating normal embryos with ultrasonics during blastoderm formation. 5. It is suggested that the primary effect of the factor is upon some sub-microscopic cellular factor in an early developmental stage, which has an effect upon ectodermal differentiation.