Figure 1.

Expression of amphioxus genes of inductive signals during late gastrula stage. Anterior is perpendicular to the plane of the page in the blastopore view and at the top in the dorsal view. Dorsal is up in the blastopore view. Scale bar, 50 μm. Amphioxus BMP2/4 is expressed throughout the ectoderm and mesendoderm, except the dorsal axial mesoderm and ectoderm, and the BMP antagonist Chordin is expressed in complementary domain in the most dorsal-medial mesendorem and ectoderm. Fgf8/17/18 is expressed in the dorsal axial mesendoderm underlying the prospective neural plate, and Wnt8 is expressed in the paraxial mesoderm and ventral mesendoderm. Notch is expressed throughout the entire mesendoderm and in the prospective neural plate, and its ligand Delta is expressed in the paraxial dorsal mesoderm.