The zygotic expression of only a few Drosophila genes is known to be required for completion of the normal embryonic mitoses. Molecular genetic analyses of these genes reveal that they fall into two classes, those whose mRNA levels are regulated in a stage and/or tissue-specific fashion to control cell cycle events during embryogenesis, and those in which, in the absence of functional zygotic expression, the maternal mRNA contribution does not provide sufficient product to complete the normal embryonic mitoses. Genes that comprise the first class are involved in the developmental control of the cell cycle, while those of the second class identify components of the cell cycle machinery.