Snail1a and Snail1b cooperate in the directed anterior migration of the axial mesendoderm. (A-D) Morphological appearance of control embryos (A), and of embryos injected with snail1a MOs (B), with snail1b MOs (C) or with both (D). The expression of hgg1 (green) and snail1b (red) is superimposed. Note the abnormal position and/or shape of the prechordal plate. The arrows mark the territory occupied by the polster. (E-H) High-magnification confocal microscopy images of the same embryos at the regions indicated with a red square in A-D. Red arrows indicate the aberrant position of groups of snail1b-expressing cells, and green arrows that of hgg1-expressing cells. (I-L) Diagrams summarising the phenotypes observed in the snail1 morphants. (J) As previously described, the injection of snail1a MOs altered the posterior position of the prechordal plate (shown in green, with the extent indicated by a double-headed arrow). (K) snail1b morphants developed an elongated prechordal plate, which reached the normal anterior position. (L) Co-injection of snail1b and snail1a MOs produced a compound phenotype in the prechordal plate, which adopted an elongated shape and was situated at an aberrant posterior position. Black arrowheads indicate the anterior and posterior limits of the embryo. The double-headed arrows on the outside of the embryos mark the distance between the embryo limits. (M-P) hgg1 in situ hybridisation and DAPI (nuclei) staining show the cell density in the regions indicated with a dotted blue square in I-L. All embryos in this figure are at tail bud stages.