Fig. 2

myoD is not expressed in the spt^-;flh^- midline. (A-D) Dorsal views of myoD expression at early segmentation stages (2-4 somites, 10.7-11.3 h, anterior to the top). In wild-type embryos (A), myoD is expressed in a two bilateral rows of adaxial cells and more laterally in each somite (Weinberg et al., 1996); in flh^- embryos (B), myoD is expressed in the midline (Halpern et al., 1995). There are fewer myoD-expressing cells in spt^- (C; Weinberg et al., 1996) and spt^-;flh^- (D) embryos, but they are confined to mesoderm flanking the developing notochord. (E-H) Dorsal views of myoD expression at late segmentation stages (26 somites, 22 h, anterior to the left). In wild-type embryos (E; Weinberg et al, 1996), myoD is expressed in bilateral rows of muscle that flank the notochord. In flh^- embryos (F), myoD is expressed in midline mesoderm. myoD expression in spt^- (G) and spt^-;flh^- (H) embryos is strikingly similar, although occasionally a few myoD-positive midline cells are observed in the double mutant tail. Scale bars, 100 um.