1. School of Veterinary Medicine, Rakuno Gakuen University, Bunkyodai 582, Ebetsu, Hokkaido 069-8501, Japan
2. Matsumoto, M., National Institute of Animal Industry, Tsukuba Norin-danchi, Ibaraki 305-0901, Japan
3. Tohoku National Agricultural Experiment Station, Kuriyagawa, Morioka 020-0123, Japan