six3 expression in the head ectoderm. Dorsoanterior views (A-C, posterior to the top) and corresponding transverse sections (D-F). Dotted lines in A-C indicate approximate levels of the sections. (A,D) Two-somite stage; six3 is expressed in both ectodermal and mesendodermal cells. (B,E) Six-somite stage; six3 mRNA is present primarily in ectodermal cells. (C,F) 16-somite stage; six3 expression is restricted to the eye and the rostral regions of the forebrain. The section shows that six3 expression in both retina and lens is dominant in surface cells, while the neural tube is largely six3 negative. Abbreviations: ec, ectoderm; l, lens; me, mesendoderm; nr, neural rod; nt, neural tube; r, retina. Scale bars, 100 μm.