1. Division of Animal Science, Department of Bioresource Science, Graduate School of Agricultural Science, Kobe University, Kobe 657-8501, Japan
2. Department of Obstetrics and Gynecology, Shiga University of Medical Science, Otsu 520-2192, Japan
3. Northern Center of Agricultural Technology, General Technological Center of Hyogo Prefecture for Agriculture, Forest and Fishery, Asago 669-5254, Japan