Fig. 4. Neuromeric model of the early embryonic brain. Based on the expression of the segment polarity genes en/inv and hh as well as the DV patterning genes msh and vnd we propose the pregnathal brain to consist of four neuromeres. Red lines indicate the borders between the tritocerebrum (Tc; comprising about 13 NBs), the deutocerebrum (Dc; comprising about 21 NBs), the ocular part of the protocerebrum (Oc-Pc; comprising about 60 NBs) and the labral part of the protocerebrum (Lr-Pc; comprising about 10 Inv-positive NBs). Note, the ventral part of the posterior border of the deutocerebrum is given by Dv2 and Dv4, which at stage 10 (st10) transiently express vnd. The neuromeric identity of Ppd2 is unclear; colour intensities indicate low () and high (+) expression levels of en/inv and hh. AN, IC, MD, MX, antennal, intercalary, mandibular and maxillary segment, respectively; CL, clypeolabrum; Dc, deutocerebrum; Lr-Pc, labral part of the protocerebrum; Oc-Pc, ocular part of the protocerebrum; Tc, tritocerebrum; FG, foregut; ML, ventral midline; OA, Bolwig organ/optic lobe anlagen.