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a Geometry of a larval-stage wing disc (Widmann and Dahmann 2009)—A anterior, P posterior, D dorsal, V ventral, DP disc pouch. b Side view along B in (A), c the columnar cells. c, d transport processes that affect the Dpp distribution: diffusion in the around columnar cells (c) or transcytosis through columnar cells (d). From (Othmer et al. 2009) with permission
