1. Department of Biological Sciences Tokyo Institute of Technology Yokohama 226‐8501Japan
2. Biomedical Engineering Center Toin University of Yokohama Yokohama 225‐8503Japan
3. Department of Biomedical Engineering Toin University of Yokohama Yokohama 225‐8503Japan
4. Graduate School of Life and Environmental Sciences University of Tsukuba Ibaraki 305‐8572Japan
5. Genome Information Research Center Research Institute for Microbial Diseases Osaka University Osaka 565‐0871Japan
6. Germ Cell Conservation Laboratory National Institute of Agrobiological Sciences Ibaraki 305‐8602 Japan