Figure 1.

General organization and presumed homologous regions of the central nervous system (CNS) in ascidian larvae and vertebrate embryos. Drawings of a Ciona larva (A), ~18 h after fertilization and a mouse embryo (B), ~9 days old (stage E9.5). The developing nervous systems of these divergent chordates exhibit a comparable anterior-posterior sequential expression of the genes Otx (yellow), Pax2/5/8 (orange), Hox1 (magenta), Hox3 (blue), and Hox5 (green). Abbreviations: ANR, anterior neural ridge; ant., anterior; AO, adhesive organ; D, diencephalon; dor., dorsal; hp, hypothalamic prosomere; MHB, mid-hindbrain boundary; Oc, ocellus; Ot, otolith; p, prosomere; pos., posterior; r, rhombomere; SP, secondary prosencephalon; TEL, telencephalon; ven., ventral; ZLI, zona limitans intrathalamica. Adapted from [10,17,55].