This paper discusses cytological changes which occur during anuran metamorphosis in a pair of large hind-brain neurones, the Mauthner cells or M-cells which, in many teleosts and amphibians, innervate the tail musculature via ventral horn cells. The M-cells of fishes, urodeles and anuran larvae are unusually large neurones of particular interest, and according to Stefanelli (1951) they constitute a 'true functional system of nervous activity'.