1. Department of Veterinary Surgery and Obstetrics, Faculty of Agriculture, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113, Japan
2. Department of Animal Reproduction, National Institute of Animal Industry, P.O. Box 5, Norindanchi, Tsukuba, Ibaraki 305, Japan
3. Department of Animal Reproduction, faculty of Agriculture, Tokyo University of Agriculture, Sakuragaoka, Setagaya-ku, Tokyo 156, Japan