During the course of this work attention must also be directed, as occasion arises, to such other bacteria, pathogenic or saprophytic, as are allied to the particular organisms under observation, or so resemble them as to become possible sources of error, by working them through on parallel lines--in other words the various bacteria should be studied in "groups." In the following pages the grouping in use in the author's elementary classes for medical and dental students and for candidates for the Public Health service is adopted, since a fairly long experience has completely vindicated the value and utility of this arrangement, and by its means a fund of information is obtained with regard to the resemblances and differences, morphological and cultural, of a large number of bacteria. The fact that some bacteria appear in more than one of these groups, so far from being a disadvantage, is a positive gain to the student, since with repetition alone will the necessary familiarity with the cultural characters of important bacteria be acquired. The study of the various groups will of course vary in detail with individual demonstrators, and with the student's requirements--the general line it should take is indicated briefly in connection with the first group only (pages 410-411). This section should be carefully worked through before the student proceeds to the study of bacterioscopical analysis. It is customary to commence the study of the pathogenic bacteria with the Organisms of Suppuration. This is a large group, for all the pathogenic bacteria possess the power, under certain conditions, of initiating purely pyogenic processes in place of or in addition to their specific lesions, (e. g., Bacillus tuberculosis, Streptococcus lanceolatus, Bacillus typhosus, etc.). There are, however, a certain few organisms which commonly express their pathogenicity in the formation of pus. These are usually grouped together under the title of "pyogenic bacteria," as distinct from those which only occasionally exercise a pyogenic rôle. The organisms included in this group are: 1. Staphylococcus pyogenes albus. 2. Staphylococcus pyogenes aureus. 3. Staphylococcus pyogenes citreus. 4. Streptococcus pyogenes longus. 5. Micrococcus tetragenus. 6. Bacillus pyocyaneus. 7. Bacillus pneumoniæ. and in certain special tissues 8. Micrococcus gonorrhoeæ. 9. Micrococcus intracellularis meningitidis (Meningococcus). 10. Micrococcus catarrhalis. 11. Bacillus ægypticus (Koch-Weeks Bacillus). The group may with advantage be subdivided as indicated in the following pages: I. _Pyogenic cocci._ Staphylococcus pyogenes albus. Staphylococcus pyogenes aureus. Staphylococcus pyogenes citreus. to contrast with Micrococcus candicans. Micrococcus agilis. 1. Prepare subcultivations from each: