Differential and staining of cobalt-and nickel-filled neurones using rubeanic acid

A staining procedure is described with differentiates with distinct colours, cobalt and nickel ions introduced into neurones. Densely coloured precipitates are produced with these ions when rubeanic acid, a spot-test reagent, is added. Formation of the coloured complexes takes place within a few minutes, and may be followed under the dissecting microscope. This technique permits observation of the relative positions, within ganglia, of somata related to two emergent nerves following multiple axonal-backfilling. It also appears to be of value in tracing neurone branching after the intracellular injection of ions.