A. ELAV-labelling of differentiating photoreceptor neurons in the eye imaginal disc. Anterior to the left. Bar indicates columns 1–4, where cells progress through the Second Mitotic Wave cell cycle. ELAV protein is first detected in column 2. GMR-GAL4 drives UAS-reporter gene expression in all cells in column 1 and more posteriorly. Progressive addition of neural cells to each ommatidial cluster occurs more posteriorly (rightwards). B. GMR-GAL4>UAS-RasV12. Ectopic neural differentiation is first detected in column 5 (arrows), after the SMW. C. At 29°C, GMR-GAL4>UAS-RasV12 is expected to drive transgene expression at a higher level. Ectopic differentiation is nevertheless restricted until after the SMW, however (arrow). D. RasV12 expression in response to 'strong' GMR-GAL4 likewise affects differentiation only posterior to column 5 (eg arrow). E. 5 h after a 10 min heat shock, hs-RasV12 has not yet affected the pattern of ELAV expression. F. 9 h after a 10 min heat shock, hs-RasV12 has induced ectopic ELAV expression both anterior to the furrow, and posterior to column 7(arrows). G. 14 h after a 10 min heatshock, ectopic ELAV expression (arrows) is observed anterior and posterior to a band of ommatidia that remain little affected. H. More intense Ras activity, due to longer heat shock, leads to general neurogenesis. 14 h after a 1 h heat shock. I. More intense Ras activity, due to transcription from the Sev promoter in SavRasV12, also induces neurogenesis in cells occupying columns 3–5.