Alleles, Animals, Antibodies, Monoclonal, Bromodeoxyuridine, Gene Expression Regulation, Developmental, Genetic Complementation Test, Homeodomain Proteins, Immunohistochemistry, In Situ Hybridization, Membrane Proteins, Mutation, Nerve Tissue Proteins, Nervous System, Neural Crest, Neurons, Phenotype, Plasmids, RNA, Receptor, Notch1, Receptors, Notch, Signal Transduction, Somites, Time Factors, Xenopus, Zebrafish, Zebrafish Proteins