Comparison of CNS regions and gene expression domains among chordates, hemichordates, annelids, and arthropods. Focus is on the anteroposterior regionalization of the anterior CNS, including the brain. The schemes reflect the situation in the CNS of the vertebrate mouse (at about embryonic day 10-12.5), the ascidians Ciona and Halocynthia [11, 13, 43, 44, 54], the appendicularian Oikopleura (at the late hatchling stage) , the amphioxus Branchiostoma (at the 10-13 somite stages) [15, 16, 45, 76], the hemichordate Saccoglossus (at the one gill slit stage) [48, 49], the polychaete annelid Platynereis (neuroectoderm at the metatrochophora stage) [46, 47], and the arthropod Drosophila (at stage 11, st11; this study). Expression of Otx/otd, Pax2,5,8, Hox1/lab, Hox5/scr, Hox6/Antp, and Hox7/Ubx genes is indicated according to the colour code. The dashed line in red indicates the interface between Otx/otd and Gbx/unpg expression domains. The expression of further genes within the gap (encircled in yellow) between the anterior Otx/otd and posterior Hox1/lab domains is noted: '+' indicates expression of the respective gene; '-' absence of expression; and '?' expression is not yet determined. The phylogenetic tree is based on . For further details, see the text. IZ, intervening zone; MHB domain, midbrain/hindbrain boundary domain; NR, neck region; Parap., Parapodial; Tent., Tentacular.