(A) The wing primordium (left) is subdivided into anterior (A) and posterior (P) compartments, as well as into dorsal (D) and ventral (V) compartments, which will give rise to specific biological structures within the adult wing (right). The compartments are named after the position that their cells and progeny will occupy by the end of development. (B, C) Early in development (B), Serrate (Ser) signals to V cells to activate Notch (N). Likewise, Delta (Dl) signals to D cells to activate Notch modified by Fringe (Fng) along the DV boundary. Later in development (C), ligands expression becomes symmetric with respect to the boundary and a positive feedback-loop between Wingless (Wg) and Ser/Dl-expressing cells maintains the signaling center along the DV boundary. Notch activity elicits Cut expression that represses Dl and Ser in boundary cells.