Relationship of rtk3 expression to the presumptive notochord. Whole-mounted embryos at 100% epiboly labelled with an antisense RNA probe to rtk3 (A-C) and an antibody to Ntl (B,C). Anterior is to the left. (A) Flat-mounted preparation. rtk3 transcripts are present at high levels in axial tissue anteriorly and paraxial tissue posteriorly. Some expression is also detectable in paraxial tissue of the head (arrowhead). (B) Posterior end of the embryo. rtk3 is expressed in paraxial mesoderm adjacent to the Ntl-containing cells of the presumptive notochord. C) Hindbrain region. The most anterior Ntl-containing cell (arrowhead) is within the posterior axial expression domain of rtk3. Abbreviations: p, pillow; pch, prechordal hypoblast; pm, paraxial mesoderm; pn, presumptive notochord; yp, yolk plug. Scale bars: 25 µm.