1. The Biological Laboratories, Harvard University Cambridge, Massachusetts 02138, U.S.A.; Present Address: Department of Zoology, University of California, Berkeley, California 94720, U.S.A.
2. The Biological Laboratories, Harvard University Cambridge, Massachusetts 02138, U.S.A.; Present Address: Laboratoire de Neurophysiologie Cellulaire du C.N.R.S., 4, Avenue Gordon-Bennett, Paris 16
3. The Biological Laboratories, Harvard University Cambridge, Massachusetts 02138, U.S.A.