1. Department of Biological Science; Graduate School of Science, Hiroshima University; Higashihiroshima 739-8526 Japan
2. Division of Embryology and Genetics; Institute for Amphibian Biology, Graduate School of Science, Hiroshima University; Higashihiroshima 739-8526 Japan
3. Department of Chemistry; Faculty of Science, Hokkaido University; Sapporo 010-0810 Japan