Fig. 3. Stages of notochord development affected by mutations. The transition from dorsal organiser to chordamesoderm is affected by mutations in several genes. At the top of the dorsal specification cascade are ichabod and ß-catenin proteins (Kelly et al., 2000). In their absence, ventral types of mesoderm can form but not chordamesoderm. The bozozok and floating head loci each encode homeodomain-containing proteins that are necessary for chordamesoderm specification. In the complete absence of oep, as in the maternal-zygotic (Mz) mutant, mesendoderm is not specified, including the chordamesoderm. The laminins (bashful, grumpy and sleepy), the coatomers (sneezy, happy and dopey), brachyury (no tail) and one unknown locus, doc, are all required for proper notochord differentiation. Arrowheads indicate (A) chordamesoderm progenitor region of the shield, (B) the chordamesoderm and (C) the differentiated notochord.