The Drosophila six3 / opti x-expressing region includes neurosecretory centres. The neuroectodermal domains of the Drosophila neurosecretory pars intercerebralis (PI) and pars lateralis (PL) lie within the six3/optix-expressing region. (a, a', a'') Six3/Dchx1 protein expression. Six3 is detected in the neuroectoderm of the developing PI, as is specifically indicated by the expression of Dchx1. (b, b', b'') Six3/Fas2 protein expression. Six3 is additionally found to be expressed in the neuroectodermal placode of the developing PL, as is indicated by the strong expression of Fas2 . (c, c') six3 mRNA/Crumbs protein expression. (c') Higher magnification of the six3-expressing head region. Black arrowheads in (c') depict invaginating placodal cells of the PI (1) and PL (2) as visualized by apically concentrated localisation of the Crumbs protein ; as is indicated by the red dots in (d). (d) Schematic summary of the expression of Six3, Dchx1, Fas2, and Crumbs in the anterior-dorsal head ectoderm, including the neuroectodermal placodes of the PI and PL, as is depicted by the colour code. LR = labrum; NE = neuroectoderm; OL = optic lobe anlagen; PI = pars intercerebralis; PL = pars lateralis.