IN 1950 Armstrong1 found that the period after injury during which a nerve fibre preserves a normal histological appearance may be surprisingly long in a poikilothermic animal kept at room temperature (15°–20° C.), but that the rate of degeneration is increased and the time after which the fibre eventually disappears is shortened by a comparatively small rise in temperature. This temperature effect is important, since one of the commonest methods for investigating pathways and connexions in the nervous system involves the interpretation of degenerative changes which follow experimental injury, and it is essential to know how long one must wait before it is possible to say that surviving nerve fibres, normal in appearance, cannot have been affected by the injury.