The brain anlagen in Drosophila is tripartite. (a) Flat preparation of the head of an Otd/Labial antibody double stained embryo at stage 11. (b) Close-up of the region framed in (a) at the level of brain NBs. (c) Schematic summary of the expression of both genes in identified brain NBs in the left hemisphere at late stage 11, corresponding to the boxed area in (d); red dashed lines indicate neuromeric boundaries. An intervening zone can be identified between the domains of Otd and Lab expression, in which both genes are expressed neither in the peripheral ectoderm (yellow arrowheads in (a)) nor in the deriving NBs (hatched area in (b,c)). Note that in the deutocerebrum (DC) some anterior NBs (Dv5,6, Dd2,3,6) express Otd, and some posterior NBs (Dv2,4) express Labial (NB nomenclature according to ). (d) Segmental topography of the Drosophila embryo at the phylotypic stage of development (stage 11); flat preparation (anterior to the top), the head capsule has been opened dorsally. The pregnathal (labral (LR), ocular (OC), antennal (AN), intercalary (IC)) and gnathal head segments are indicated on the right side. On the left side, the primordium of the CNS is outlined (PC, protocerebrum; DC, deutocerebrum; TC, tritocerebrum; MD, mandibular, MX, maxillary, and LA, labial neuromere, respectively). Abbreviations: a, d, p, v, anterior, dorsal, posterior, ventral; AN, antennal appendage; CL, clypeolabrum; FG, foregut; ML, midline; VNC, ventral nerve cord.