Movies 1 and 2. Time-lapse movies of the simulation without (Movie 1) and with (Movie 2) Ptc negative feedback regulation. Upper panel shown the concentration of Hh across the domain. Lower panel shows the cellular concentration of Ptc (green), Sens (red) and Elav (blue). Despite the variable response to Hh due to cell variability, a wave in photoreceptor differentiation (blue cells) can be observed as traveling away from the Hh source. The first movie (no Ptc downregulation) shows that the wave velocity diminishes and stops before reaching the end of the domain. The Hh gradient does not flatten. The second movie includes Ptc negative feedback, and shows how the differentiation wave moves at constant speed and reaches the end of the domain. The Hh gradient flattens.