Fig. 2. Spatial and temporal development of the embryonic brain NB pattern. (A-G) Semi-schematic representations of ventral views on the left half of head flat preparations, double stained for svp-lacZ and Engrailed (en-lacZ or anti-Invected antibody) at (A) mid/late stage 8 (m/lst8), (B) early stage 9 (est9), (C) late stage 9 (lst9), (D) early stage 10 (est10), (E) mid stage 10 (mst10), (F) early stage 11 (est11) and (G) late stage 11 (lst11). Expression of svp-lacZ and engrailed (en) in NBs is indicated in blue and red, respectively; newly formed NBs at each stage are in yellow; red hatching marks en expression in the peripheral ectoderm. Anterior (a) is towards the top and dorsal (d) is towards the left. Note, most of the depicted stages correspond to those described as phases `S1-S5' for the pattern of NB formation in the trunk (Doe, 1992): m/lst8 corresponds to early S1; est9 to S1; lst9 to S2; e/mst10 to S3; est11 to S4; and lst11 to S5 phase. In addition to the pregnathal segments, the evolving NB pattern is also shown for the mandibular segment (MD; asterisks indicate mandibular NBs). At early/mid stage 9 (B), when svp expression initiates, it is detected in a simple pattern, including about seven protocerebral and all deutocerebral NBs. At late stage 9 (C), two En-positive deutocerebral NBs (Dd5 and Dd13) derive from the en antennal stripe (as), and one protocerebral NB (Ppd5) from the en head spot (hs). New NBs form at the borders of the developing NB array, but in addition, individual NBs become integrated at various positions into the pre-existing NB pattern (D-G). By late stage 11 (G), Inv (but not en-lacZ) is weakly detected in the anteriormost procephalon (dh); the faint Inv expression in about 10 NBs deriving from the dh is not indicated (see Urbach and Technau, 2003a). Note that the formation of NBs in the intercalary (IC) and (anterior) mandibular segment (MD) is significantly delayed. Formation of tritocerebral NBs starts at stage 10. (H) Fully developed NB array (lst11) with the stage of formation indicated for each cell (see key). Most of the early (stage 8/early stage 9) NBs occupy central parts of the protocerebral primordium at different D/V positions. a, d: anterior, dorsal; as, is, mds: antennal, intercalary and mandibular en stripe, respectively; cl: en expression in the clypeolabrum; dh: en expression in the dorsal hemispheres; hs: en head spot; AN, IC, MD, MX: antennal, intercalary, mandibular and maxillary segment, respectively; CL: clypeolabrum; FG: foregut; ML: ventral midline; OA: Bolwig organ/optic lobe anlagen.