Fig. 6. Schematic representation of relationships between somites, anterior expression boundaries of some Hox genes and vertebrae. Anterior expression boundaries of zebrafish Hox genes [names according to Amores et al. (Amores et al., 1998)] are primarily as in ¹van der Hoeven et al. (van der Hoeven et al., 1996) ²Prince et al. (Prince et al., 1998a) and ³Sordino et al. (Sordino et al., 1996). Differing anterior boundaries indicated by stripes have been reported (Sordino et al., 1996; Prince et al., 1998a) for hoxa10b. Zebrafish hoxc6 homologs, with anterior expression boundaries at somite 5, align with the transition between cervical and thoracic (rib-bearing) vertebrae as in chick and mouse (Burke et al., 1995). By contrast, the anterior expression boundaries of zebrafish Hox paralog group 9 genes fall within the anterior thoracic domain, as apposed to Hox paralog group 9 alignment with the thoracic/lumbar transition zone in chick and mouse (Burke et al., 1995). The hoxd12a anterior expression boundary is the posterior-most described and yet also falls within the region that will contribute to rib-bearing vertebrae. Posterior somites are indicated by stripes because the resolution of total somite and vertebral numbers remains unclear.