4. _Nucleus._--This structure has not been conclusively proved to exist, but in some bacteria chromatin particles have been observed near the centre of the bacterial cell and denser masses of protoplasm situated at the poles which exhibit a more marked affinity than the rest of the cell protoplasm for aniline dyes. These latter are termed polar granules or _Polkoerner_ (Fig. 85, e). Occasionally these aggregations of protoplasm alter the colour of the dye they take up. They are then known as metachromatic bodies or _Ernstschen Koerner_ (e. g., B. diphtheriæ). 5. _Flagella_ (Organs of Locomotion, Fig. 85, a).--These are gelatinous elongations of the cell protoplasm (or more probably of the capsule), occurring either at one pole, at both poles, or scattered around the entire periphery. Flagella are not pseudopodia. The possession of flagella was at one time suggested as a basis for a system of classification, when the following types of ciliation were differentiated (Fig. 87): [Illustration: FIG. 87.--Types of ciliation.] 1. Polar: (a) _Monotrichous_ (a single flagellum situated at one pole; e. g., B. pyocyaneus). (b) _Amphitrichous_ (a single flagellum at each pole; e. g., Spirillum volutans). (c) _Lophotrichous_ (a tuft or bunch of flagella situated at each pole; e. g., B. cyanogenus). 2. Diffuse: _Peritrichous_ (flagella scattered around the entire periphery e. g., B. typhosus). ~PHYSIOLOGY.~ ~Reproduction.~--_Active Stage._--Vegetative, i. e., by the division of cells, or "fission." 1. The cell becomes elongated and the protoplasm aggregated at opposite poles. 2. A circular constriction of the organism takes place midway between these aggregations, and a septum is formed in the interior of the cell at right angles to its length. 3. The division deepens, the septum divides into two lamellæ, and finally two cells are formed. [Illustration: FIG. 88.--Fission of cocci.] [Illustration: FIG. 89.--Fission of bacteria.] 4. The daughter cells may remain united by the gelatinous envelope for a variable time. Eventually they separate and themselves subdivide. Cultures on artificial media, after growing in the same medium for some time--i. e., when the pabulum is exhausted--show "involution forms" (Fig. 90), well exemplified in cultures of B. pestis on agar two days old, B. diphtheriæ on potato four to six days old. [Illustration: FIG. 90.--Involution forms.] They are of two classes, viz.: (a) Involution forms characterised by alterations of shape (Fig. 90). (Not necessarily dead.) (b) Involution forms characterised by loss of staining power. (Always dead.) _Resting Stage._--Spore Formation.--Conditions influencing spore formation: In an old culture nothing may be left but spores. It used to be supposed that spores were _always_ formed, so that the species might not become extinct, when (a) The supply of nutrient was exhausted. (b) The medium became toxic from the accumulation of metabolic products.