Anterior–posterior gene expression in hemichordate ectoderm and central nervous systems of three chordate subphyla. Anterior–posterior regionalization of gene expression domains in the ectoderm of the hemichordate Saccoglossus kowalevskii as well as in the central nervous system (CNS) of representatives of the three chordate subphyla (that is, amphioxus, ascidian tunicates and vertebrates). Question marks on the diagrams of the amphioxus and ascidian CNS indicate that organizer properties of the regions marked ‘ZLI?’ and ‘ANR?’ have not been tested. AP, anterior proboscis; PB, proboscis; PCB, proboscis/collar boundary; COL, collar; CTB, collar/trunk boundary; TR, trunk; ANR, anterior neural ridge; ZLI, zona limitans intrathalamica; MHB, midbrain/hindbrain boundary; SV, sensory vesicle; N, neck; G, ganglion; ISO, isthmic organizer; Tel, telencephalon; Di, diencephalon; Mes, mesencephalon. Gene expression domains based on [9, 24, 34, 64, 68–72, 74–80].