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Fig. 5 Dynamic regulation of spt expression in ntl- embryos. (A,B) Mid gastrula (7.5 h.p.f., vegetal view). spt is expressed at normal levels in the ntl mutant embryo. Although the spatial distribution of spt expression is slightly different in ntl- embryos, the broader region of non-expressing cells represent dorsal mesoderm as they express the axial mesoderm marker floating head (data not shown). (C,D) 8-somite stage (13 h.p.f., lateral view, anterior is left). spt expression in the tail bud epiblast (arrow) is readily detected in WT embryos (C), but this expression domain is not seen in ntl- embryos at a similar stage (D). The domain of spt expression in involuted mesoderm is only partially reduced in the mutant embryo. The increased thickness of the epiblast in ntl- embryos is already apparent at this stage (dots mark outer surface of epiblast).