[en] Development of an effective method to produce γ-ray induced mutants was attempted using medaka, a killifish of which development has been analyzed in detail. An experimental system to make functional analysis for the gene of nerve growth factor was constructed using γ ray -induced mutants and the mechanism to induce its morphogenesis and the nerve growth common to invertebrates was investigated. Some mutants with abnormalities in the chorda dorsalis formation were constructed by exposure to γ ray. Thus, the expression of HNF3β gene which is likely to be involved in the formation of chorda dorsalis was investigated and it was demonstrated that this gene is not expressed in the end region of chorda dorsalis and its succeeding mesoderm. Moreover, the genes expressed at the end region of wild type killifish was isolated by differential hybridization for DNA library that was constructed based on cDNA for the mRNA produced in the range. Twenty genes homologous to the previously reported genes and 51 not homologous ones were isolated. These genes would be utilized for the investigation of their expression. (M.N.)