ABSTRACT
We organized this workshop as an open discussion around the theme of developmental organization and evolution of the vertebrate head. To sharpen the focus of the debate initially, we, the convenors, proposed a hypothetical ‘two-step’ model of head development. The first step would be conceptually analogous to insect development, in the sense that early in embryonic development, i.e. during, or shortly after, gastrulation, a segmental pattern is specified in the neuroectoderm and/or head mesoderm. By segmentation we mean the subdivision of an embryonic field into sharply defined populations of cells linearly arranged along the anterior-posterior axis. Each population of cells would correspond to a ‘segment’ and each segment would be characterized by divergent developmental fate and potency. This initial process of regionalization by segmentation would be followed by a series of morphogenetic events, involving deformation of epithelial sheets, cell migration and epithelial-mesenchymal inductive interactions.
