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ABSTRACT
Outgroup analysis of the ontogenies of extant deuterostomes allows one to recognize derived developmental events. These events indicate how an ancestral ontogeny was altered to produce the existing variation among deuterostomes. The origin of craniates is linked to the origin of neural crest and neurogenic placodes, the reorganization of cephalic paraxial mesoderm, and the muscularization of hypomeric mesoderm. New information on the embryonic fate of these tissues and the role of homeobox genes in their patterning indicates that duplication of one cluster of such genes, the Hox genes, may have repatterned a portion of the ectoderm, producing both neural crest and neurogenic placodes. Several additional families of homeobox genes appear to be involved in the genesis of the cerebral hemispheres and olfactory organs. Although the role of homeobox genes in the patterning of cephalic paraxial mesoderm and pharyngeal endoderm remains unclear, these new ontogenetic and molecular data necessitate continued re-examination of the phylogenetic processes underlying the origin of craniates.