1. National Food Safety and Toxicology Center, Department of Zoology, Institute of Environmental Toxicology, Michigan State University, East Lansing, Michigan 48824, Department of Environment Conservation, Ehime University, Tarumi 3-5-7, Matsuyama 790, Japan, and Department of Hygiene, School of Medicine, Yokohama City University, 3-9 Fukuura, Kanazawa-ku, Yokohama 236, Japan