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Fig. 3 Expression of spt in WT and spt- embryos and regulation by FGF signaling. (A) spt expression, first detected between sphere and dome stages, is ubiquitous in the late blastula embryo (40% epiboly, 5 h.p.f., lateral view). (B) By the early gastrula stage (shield, 6 h.p.f., dorsal view), spt expression is specific to the mesoderm and, in an animal pole view (C), is found throughout all regions of the germ ring (d, dorsal). (E) spt expression refines during gastrulation and by 60% epiboly (7 h.p.f., dorsal view), spt is not expressed in the notochord progenitors (n), whereas lateral and ventral germ ring and head mesoderm (arrow) express spt. (F,G) Vegetal views of spt expression in late-gastrula embryos (80% epiboly, 8.3 h.p.f., dorsal uppermost). (F) Expression of a dominant-negative FGF receptor (dnFGFR), which is mosaicly distributed (Griffin et al., 1995), causes loss of spt expression in large areas of the germ ring; compare with WT in G. (I) At the 4-somite stage (11.5 h.p.f., anterior to left) expression is localized to the hatching gland progenitors (arrow), the tail bud and the segmental plate mesoderm. (J,K) In the tail bud, spt is expressed in epiblast cells at the site of involution (arrowhead in J, * in K), and in the segmental plate (S); spt is also expressed in adaxial cells (arrows; Devoto et al., 1996). (J, 8 somites, 13 h.p.f., lateral view, anterior to left; K, 4 somites, 11.5 h.p.f., posterior view, dorsal uppermost). (D,H,L) spt expression in spt- embryos. (D) In early gastrula spt- embryos (shield, 6 h.p.f., animal pole view), spt expression is weak but has normal spatial distribution (d, dorsal); compare with C. (H) During gastrulation (80% epiboly, 8.3 h.p.f., vegetal view), spt expression is not maintained in hypoblast or dorsolateral epiblast, but is weakly expressed on the ventral side; compare with G. (L) In early-somite stage spt- embryos (4 somite, 11.5 h.p.f., posterior view, dorsal uppermost), spt is expressed in the tail bud (*) but not in the segmental plate mesoderm (compare with K).