(A) In birds and mammals, the epidermis features a geometric patterning, with feathers or hair follicles having regular spacing between them. According to the reaction-diffusion model, this organisation emerges because of short-range activators (yellow) and long-range inhibitors (blue) limiting the sites at which feathers or hair follicles form. (B) During the larval stage, the cells that will form structures in the adult fly develop in sacs of cells called imaginal discs, which can be identified by their shape and position in the larva. Colour-coding shows the correspondence between individual discs and adult structures (e.g. the red imaginal discs in the larvae will mature into eyes – also in red). Disc dissection, culture, and direct imaging allow visualization of developmental patterning processes, as shown by Gallagher and colleagues for the eye disc. (C) Gallagher et al. propose an updated model to explain the geometric patterning of the eye disc. An activator (atonal, yellow), and Scabrous (a possible secreted inhibitor, blue) are expressed by early photoreceptor clusters. The yellow stripe on the left represents the morphogenetic furrow moving from the posterior (right) to the anterior side of the disc (left). Cells to the left of the furrow constrict and those to the right expand, driving cell flow in the pattern shown by the black arrows. This cellular movement ensures regularly spaced photoreceptor clusters.