Figure 2. Drosophila ommatidial development

1. Morphogenetic furrow (purple) is propagated from posterior to anterior (bottom to top) across the eye disc by Hedgehog (black arrows), expressed by developing photoreceptors (gray). 2. atonal expression (black) becomes restricted to R8 photoreceptor by inhibitory interactions (red arrows). 3. Progressive recruitment of non-R8 photoreceptors (blue) by inductive/permissive interactions (green arrows).