1. K. Itoh, B. Stevens, R. D. Fields, National Institutes of Health, National Institute of Child Health and Human Development, LDN 49/5A38, Neurocytology and Physiology Unit, Bethesda, MD 20892, USA.
2. M. Schachner, Department of Neurobiology, Swiss Federal Institute of Technology, Honggerberg, 8093 Zurich, Switzerland.