Mechanism of restoring gradient continuity. Clones and the ensuing discontinuities in Dpp signalling or Omb levels are symbolized by dotted contours. (A) Clones lacking Dpp signaling generate sharp discontinuities in the Dpp signaling gradient. Extrusion of mutant cells restores the monotonous decline of the gradient. (B) Manipulations of Omb level. Numbers next to the dotted bars refer to the experimental manipulation by which changes in Omb level were effected. (1) Upregulation of Omb in lateral tkvQDclones. (2) Loss of Omb in omb null mutant clones (strong Omb reduction by ombRNAi was similarly effective in eliciting cellular retraction). (3a) Clone of cells (tub>omb) strongly overexpressing Omb and surrounded by wild type cells. (3b) Group of wild type cells surrounded by cells strongly overexpressing Omb. In the bottom diagrams, presenting x-z views of mosaic wing discs, clonal cells are drawn in green.