A-C. Normal pattern of neurogenesis and cell cycle progression in GMR-GAL4. Cells enter S-phase of the SMW after column 0, and most cells (~90%) perform mitosis and degrade their Cyclin B between columns 3–5. Arrowhead indicates column 0 in the morphogenetic furrow. D-F. In GMR-GAL4>RasV12, ectopic neurogenesis occurs after the Second Mitotic Wave is completed (arrow). 100% of SMW cells perform mitosis and degrade Cyclin B in this genotype. G-I. GMR-p21 prevents entry into the SMW, so that no cells contain Cyclin B posterior to the morphogenetic furrow . Ectopic neurogenesis is still delayed until after column 5, however (arrow).