Fig. 7. Schematic description of how the pigment rim is formed. (A) Upper panel shows the presumptive retina at three stages of the third instar larva. Towards the left, the early retina is flanked by the Wg-secreting presumptive head capsule (HC, blue) and the Hh wave is incipient (green arrows). In the middle and to the right; as the wave sweeps the retina, the antagonistic Wg signal (blue t-stops) prevents ommatidia from differentiating close to the HC. (B) Lower panel shows a high power view close to the HC during the pupal phase. Left: the strip immediately adjacent to the HC is occupied by presumptive pigment cells because ommatidial differentiation was inhibited there. The outer two rows of ommatidia express Hth, thereby specifying them as DRO, but the most peripheral row also receives the Wg signal (blue arrows) that indirectly causes their apoptosis. Middle: the peripheral ommatidia die. Right: the pigment cells that surrounded the dying ommatidia now join the peripheral pigment cells to form the PR, and the most peripheral ommatidia are now the surviving DRO units.