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Fig. 6 spt function regulates tbx6 expression in trunk but not tail mesoderm. (A) In WT embryos at the early gastrula stage (shield, 6 h.p.f., animal pole view; d, dorsal) tbx6 expression is restricted to ventral mesoderm. During gastrulation (B; 80% epiboly, 8.3 h.p.f., vegetal view) tbx6 expression is up-regulated in the dorsolateral mesoderm, and during segmentation (C,D) tbx6 is extensively expressed in tail bud epiblast (*) and segmental plate (C, 2-4 somites, 11 h.p.f.; D, 7-8 somites, 13 h.p.f.; posterior view, dorsal uppermost). In spt- embryos, tbx6 expression is initially weak (E), remains restricted to the ventral side during gastrulation (F) and is barely detectable in the segmental plate at 2-4 somites (G). However, at 7-8 somites (H) tbx6 has a relatively normal distribution compared to WT, and is easily detected in segmental plate mesoderm (arrows) in addition to the tail bud (*).