1. Division of Developmental Biology, Research Center for Marine Biology, Tohoku University, Asamushi, Aomori 039-3501, Japan
2. Present address: Nara Institute of Science and Technology, Laboratory of Neuronal Cell Morphogenesis, Graduate School of Biological Sciences, Ikoma 630-0192, Japan
3. Present address: Tokyo University of Marine Science and Technology, Field Science Center, Tateyama Station (Banda), Chiba 294-0308, Japan
4. Marine and Coastal Research Center, Ochanomizu University, Tateyama, Chiba 294-0301, Japan