1. Laboratory of Veterinary Anatomy and Developmental Biology, The United Graduate School of Veterinary Science, Yamaguchi University, Yamaguchi 753-0841, Japan
2. Department of Biomedicine, Graduate School of Medical Science, Kyushu University, Fukuoka 812-8582, Japan
3. Laboratory of Applied Genetics, Graduate School of Agricultural and Life Science, The University of Tokyo, Tokyo 113-8657, Japan