1. Precursory Research for Embryonic Science and Technology (PRESTO) Japan Science and Technology Agency (JST) Saitama 332‐0012 Japan
2. Department of Physiology Keio University School of Medicine 35 Shinanomachi Shinjuku‐ku Tokyo 160‐8582 Japan
3. Department of Anatomy Keio University School of Medicine 35 Shinanomachi Shinjuku‐ku Tokyo 160‐8582 Japan
4. Department of Anatomy Kawasaki Medical School 577 Matsushima Kurashiki 701‐0192 Japan