Author's Organization:Department of Veterinary Surgery and Obstetrics, Faculty of Agriculture, University of Tokyo Department of Veterinary Surgery and Obstetrics, Faculty of Agriculture, University of Tokyo Department of Veterinary Surgery and Obstetrics, Faculty of Agriculture, University of Tokyo Department of Veterinary Surgery and Obstetrics, Faculty of Agriculture, University of Tokyo Department of Veterinary Surgery and Obstetrics, Faculty of Agriculture, University of Tokyo