Leg imaginal disc from a Drosophila early third instar larva, stained to show the sectorial pattern of expression of the wingless (blue) and engrailed (brown) genes. During limb development, these genes are expressed in adjacent sectors of the imaginal discs. These sectors provide positional references for a polar coordinate system that functions in the generation of patterns along the proximodistal axis. See page 484 and the Perspective on page 471. [Photograph: J. P. Couso]

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