Fig. 9. Opposing functions of Hedgehog and Wingless signalling separate midline siblings into an anterior and posterior compartment. (A) At stage 9, after the division of the midline precursors, Wingless signalling maintains Engrailed expression in two midline siblings. (B) At stage 10, repression by Wingless and activation by Hedgehog positions the proneural Lethal of scute cluster directly posterior to the early Engrailed-expressing cells. (C) Expression of Hedgehog and Lethal of scute are needed to induce and maintain late Engrailed expression in the midline siblings. (D) The induction of Engrailed by Hedgehog divides midline cells into two compartments. The absence of Engrailed in the anterior compartment allows Notch/Delta signalling to select between the MP1 interneuron and midline glial fate; Engrailed expression in the posterior compartment restricts the selection of cell fate by Notch/Delta signalling to MNB and VUM neurons. Midline cells at the parasegmental border, in the middle of the segment, may be determined as precursors.