Some fifty years ago a German investigator, Eeissner, discovered lying freely in the central canal of the spinal cord of the lamprey a very fine cylindrical rod, which he supposed, notwithstanding its unusual situation, to be a delicate nerve fibre. He failed, however, to learn anything concerning its connection with the central nervous system, and his discovery, although confirmed, seems to have attracted but little attention. The few observers who have since that time recorded observations upon Eeissner's fibre were almost all agreed that it was to be looked upon merely as an artifact produced by the coagulation of the cerebro-spinal fluid by the action of the fixing reagents employed.