_Chromoparous_ Bacteria: in which the pigment is diffused out upon and into the surrounding medium. _Chromophorous_ Bacteria: in which the pigment is stored in the cell protoplasm of the organism. _Parachromophorous_ Bacteria: in which the pigment is stored in the cell wall of the organism. Different species of chromogenic bacteria differ in their requirements as to environment, for the production of their characteristic pigments; e. g., some need oxygen, light, or high temperature; others again favor the converse of these conditions. _Light Production._--Some bacteria, and usually those originally derived from water, whether fresh or salt, exhibit marked phosphorescence when cultivated under suitable conditions. These are classed as "photogenic." _Enzyme Production._--Many bacteria produce soluble ferments or enzymes during the course of their growth, as evidenced by the liquefaction of gelatine, the clotting of milk, etc. These ferments may belong to either of the following well-recognised classes: proteolytic, diastatic, invertin, rennet. _Toxin Production._--A large number, especially of the pathogenic bacteria, elaborate or secrete poisonous substances concerning which but little exact knowledge is available, although many would appear to be enzymic in their action. These toxins are usually differentiated into-- _Extracellular_ (or Soluble) Toxins: those which are diffused into, and held in solution by, the surrounding medium. _Intracellular_ (or Inseparate) Toxins: those which are so closely bound up with the cell protoplasm of the bacteria elaborating them that up to the present time no means has been devised for their separation or extraction. _End-products of Metabolism._--Under this heading are included-- Organic Acids (e. g., lactic, butyric, etc.). Alkalies (e. g., ammonia). Aromatic Compounds (e. g., indol, phenol). Reducing Substances (e. g., those reducing nitrates to nitrites). Gases (e. g., sulphuretted hydrogen, carbon dioxide, etc.). And while the discussion of their formation, etc., is beyond the scope of a laboratory handbook, the methods in use for their detection and separation come into the ordinary routine work and will therefore be described (_vide_ page 276 _et seq._). X. NUTRIENT MEDIA. In order that the life and growth of bacteria may be accurately observed in the laboratory, it is necessary-- 1. To _isolate_ individual members of the different varieties of micro-organisms. 2. To _cultivate_ organisms, thus isolated, apart from other associated or contaminating bacteria--i. e., in _pure culture_. For the successful achievement of these objects it is necessary to provide nutriment in a form suited to the needs of the particular bacterium or bacteria under observation, and in a general way it may be said that the nutrient materials should approximate as closely as possible, in composition and character, to the natural pabulum of the organism.