Studies on the effects of some genetic lethal factors on the embryonic development ofDrosophila melanogaster : II. An analysis op the mutantX 2.

1. The development of the embryonic lethal mutantX 2 was studied by means of serial sections and by time-lapse ciné-photography of the complete development. 2. The embryo reaches a late stage of development, and survives for several hours after the time at which normal embryos hatch. Differentiation of all the tissues occurs, but the organ systems are extremely disorganized. 3. The most distinctive characteristic of the late embryo is the absence of gut in the middle region, and its partial external development at the anterior and posterior ends. 4. The abnormalities of the late embryo result from the disorganization resulting from two effects which appear at the end of gastrulation:- 1) the partial or complete failure of the endoderm and proctodaeal and stomodaeal rudiments to invaginate; 2) the presence of deep furrows over the surface of the embryo. 5. The primary effect of the factor appears to be upon the control of nuclear and cell division in blastoderm formation or in early gastrulation. This results in an excessive number of surface cells in the germ band, which causes it to become furrowed at the time of its extension. This in turn probably causes mechanical interference with the invagination of the gut rudiments.