When Lehmann and Neumann (1896) first proposed the genera Corynebacterium and Mycobacterium, the former was intended primarily for the diphtheria organism and the latter for the tubercle and leprosy organisms. In recent years there has been a tendency to broaden both of these genera to include, on the one hand, almost any species showing morphological irregularity and, on the other, various gram-positive nonsporeformers even though showing little or no irregularity in morphology. The original descriptions of these genera were very simple and included only the following essential characters: Mycobacterium. Slender rods with some branching; acid-fast; colonies on agar, dry, wrinkled. Type, M. tuberculosis. Corynebacterium. Rods with ends often swollen and club-shaped, banded with alternate streaks of stain, sometimes developing filaments and true branching (by implication non-acid-fast, although this characteristic is not definitely mentioned by the authors until a later edition of their book); growth on agar, soft and nonadherent. Type, C. diphtheriae. Various other characteristics have been listed by later authors for the genus Corynebacterium, the most important of which is the so-called "snapping division" of the cells. As this feature is difficult to observe directly, it is usually inferred from the orientation of the cells as described by Kisskalt and Berend (1918), i.e., a tendency to pile up in heaps, with palisade or V-form arrangement. Stress on this characteristic by later authors has undoubtedly been responsible for some unwarranted broadening of the genus, as orientation of this sort can often be observed and does not necessarily indicate the type of cell division which s supposed to be characteristic of Corynebacterium. As a matter of fact, broadening of the two genera has taken place in several directions until they have come to overlap. Moreover, each genus has had species assigned to it which seem to differ more from other species in the same genus than does the type species of one genus from the type of the other. This broadening has taken place along the following lines: Mycobacterium. (1) The inclusion of all acid-fast forms, whether or not branching occurs. (2) The inclusion of many branching forms (Krassilnikov, 1934) whether or not they are acid-fast. Corynebacterium. (1) The inclusion of a rapidly expanding group of "diphtheroids," i.e., animal parasites which are gram-positive and show the type of orientation described by Kisskalt and Berend; a few of these are anaerobic. (2) The inclusion of certain gram-positive plant pathogens, following the lead of
