Craniofacial development and the evolution of the vertebrates: the old problems on a new background.

Based on recent advances in experimental embryology and molecular genetics, the morphogenetic program for the vertebrate cranium is summarized and several unanswered classical problems are reviewed. In particular, the presence of mesodermal segmentation in the head, the homology of the trabecular cartilage, and the origin of the dermal skull roof are discussed. The discovery of the neural-crest-derived ectomesenchyme and the roles of the homeobox genes have allowed the classical concept of head segmentation unchanged since Goethe to be re-interpreted in terms of developmental mechanisms at the molecular and cellular levels. In the context of evolutionary developmental biology, the importance of generative constraints is stressed as the developmental factor that generates the homologous morphological patterns apparent in various groups of vertebrates. Furthermore, a modern version of the germ-layer theory is defined in terms of the conserved differentiation of cell lineages, which is again questioned from the vantage of evolutionary developmental biology.