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Fig. 4 Axial mesodermal differentiation is initiated but not maintained in flh mutants. twist is expressed by axial mesoderm in (A) WT and (B) flh mutant gastrulas at 80% epiboly (8.25 hours). At the 3-somite stage (11 hours) in (C) WT and (D) mutant embryos, twist is also expressed in the presumptive pronephric ducts. Although twist transcripts are abundant in newly-forming WT notochord at this stage (C), expression is entirely absent in the flh- axis (D). In B, flh mutant embryos were overstained approximately three-fold relative to WT sibling embryos. Overstaining of flh- embryos at early somite stages comparable to D, failed to reveal twist transcripts in the axis.