1. Laboratory of Animal Genetics, Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
2. Present address: Laboratory of Veterinary Anatomy, School of Veterinary Medicine, Kitasato University, Higashi, Towada, Aomori 034-8628, Japan
3. Laboratory of Animal Morphology and Function, Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan