1. Marine Biological Laboratory, Woods Hole, Massachusetts 02543
2. Life Sciences Centre, Dalhousie University, Halifax, Canada B3H 4J1;
3. Laboratoire de Génétique et Physiologie du Développement, IBDM, CNRS/INSERM/Université de la Méditerranée/AP de Marseille, Case 907, Campus de Luminy, F-13288 Marseille Cedex 09, France;
4. Section of Developmental Neurophysiology, Okazaki Institute for Integrative Bioscience, Nishigonaka 38, Myodaiji, Okazaki 444-8585, Aichi, Japan
5. Molecular Neurobiology Group, Neuroscience Research Institute, National Institute of Advanced Industrial Science, Higashi 1–1, Tsukuba, Ibaraki 305, Japan;