Studies on the effects of some genetic lethal factors on the embryonic development ofDrosophila melanogaster : V. An analysis of the mutantX 10.

1. The development of the embryonic lethal mutantX 10 was studied by means of serial sections and by observations on living embryos. 2. The factor varies in its expression; three distinct types of abnormal embryos are produced, and in some the lethality does not occur until the larval stage. 3. Intype I embryos development stops after the formation of a cap of undifferentiated cells; intype II, differentiation is extremely limited, often giving nervous tissue exclusively, and no organ formation occurs;type III embryos reach a late stage of development, and the main features of morphogenesis are undisturbed, but cellular differentiation is retarded and abnormal. 4. Type III embryos provide some evidence that the pole cells invaginated with the posterior mid-gut rudiment give rise to both the germ cells and to the middle mid-gut cells, and also that the mesodermal parts of the gonads do not develop in the absence of the germ cells. 5. The origin of distinct types of embryonic lethal abnormalities, and of a larval lethality, from a single mutant factor, is discussed in terms of general developmental concepts.