An embryonic cell marker system has been developed in Drosophila melanogaster that has enabled us to identify the genotype of cells as early as the cellular blastoderm stage of development. This system allows unambiguous detection of embryos homozygous for most X-linked lethal mutations at stages prior to when their first defects become obvious. By examining gynandromorphs at this stage, we have observed that the number of nuclei per unit area in male regions is about half that in female regions. An examination of early cleavage stage embryos whose DNA has been stained with Hoechst 33258 and whose actin has been stained with phalloidin suggests that this difference is due to a cell cycle delay in cells losing the ring-X. These experiments also demonstrate the existence of a mechanism which controls the timing of nuclear divisions in cycle 10-14 embryos.