The recent demonstration that many species of pathogenic bacteria "mutate" or "dissociate" into two or more morphologic variants on artificial culture mediums, and that the different pleomorphic "phases" of the same micro-organism are at times of widely different virulence and antigenicity, has introduced a new element of uncertainty into specific antibacterial therapy. For example, future clinicians may have to deal with such complexities as the "mucous membrane phase," "alveolar phase" and "septicemic phase" of the pneumococcus, and with primary, secondary and perhaps tertiary antigenic variants of numerous other specific pathogenic agents. A few such antigenic phases have been definitely established. The successive waves of relapsing fever, for example, are of qualitatively different specific antigenicities. Similar qualitative differences are well established between the primary and tertiary phases of infection with Spirochaeta pallida. Both of these micro-organisms, therefore, are definitely "diphasic" in their biochemical specificities.
Although Thomas's1 recent studies of the