man occurs through the mediation of the cerebrospinal fluid." To accomplish this transfer (that is, from blood to cerebrospinal fluid), time is required, since the barrier of the choroid plexus must first be over¬ come. At the expiration of forty-eight hours follow¬ ing the intravenous inoculation, the barrier appears still to be intact ; at the expiration of seventy-two hours the passage of the virus seems to have begun,