1. Department of Physiology, Keio University School of Medicine, Shinano‐machi, Shinjuku‐ku, Tokyo 160‐8582
2. Department of Biology, Osaka University, Toyonaka 560‐0043 and
3. Precursory Research for Embryonic Science and Technology, Japan Science and Technology Corporation, Senri Life Science Center 10F, Sin‐Senri, Toyonaka 565‐0082, Japan