The general purpose of this research is to understand how embryonic cells, all descended from the same zygote, become determined for different developmental fates. We believe that Drosophila offers exceptional advantages for studying this problem. We propose to isolate and characterize maternal-effect developmental mutants in which the eggs produced by homozygous mutant females are defective and cannot support normal development. The defective eggs will be assayed by a microinjection method for their developmental response to factors present in normal eggs. This test could provide an assay for the identification of components in the egg which are essential for normal embryogenesis. The developmental block in each mutant will also be analyzed by histological and immunological methods.