(c) The environment became unfavourable; e. g., change of temperature. This is not altogether correct; e. g., the temperature at which spores are best formed is constant for each bacterium, but varies with different species; again, aerobes require oxygen for sporulation, but anaerobes will not spore in its presence. (A) Arthrogenous: Noted only in the micrococci. One complete element resulting from ordinary fission becomes differentiated for the purpose, enlarges, and develops a dense cell wall. One or more of the cells in a series may undergo this alteration. This process is probably not real spore formation, but merely relative increase of resistance. These so-called arthrospores have never been observed to "germinate," nor is their resistance very marked, as they fail to initiate new cultures, after having been exposed to a temperature of 80° C. for ten minutes. (B) Endogenous: The cell protoplasm becomes differentiated and condensed into a spherical or oval mass (very rarely cylindrical). After further contraction the outer layers of the mass become still more highly differentiated and form a distinct spore membrane, and the spore itself is now highly refractile. It has been suggested, and apparently on good grounds, that the spore membrane consists of two layers, the exosporium and the endosporium. Each cell forms one spore only, usually in the middle, occasionally at one end (some exceptions, however, are recorded; e. g., B. inflatus). The shape of the parent cell may be unaltered, as in the anthrax bacillus, or altered, as in the tetanus bacillus, and these points serve as the basis for a classification of spore-bearing bacilli, as follows: (A) Cell body of the parent bacillus unaltered in shape (Fig. 91, a). (B) Cell of the parent bacillus altered in shape. 1. _Clostridium_ (Fig. 91, b): Rod swollen at the centre and attenuated at the poles; spindle shape; e. g., B. butyricus. 2. _Cuneate_ (Fig. 91, c): Rods swollen slightly at one pole and more or less pointed at the other; wedge-shaped. [Illustration: FIG. 91--Types of spore-bearing bacilli.] 3. _Clavate_ (Fig. 91, d): Rods swollen at one pole and cylindrical (unaltered) at the other; keyhole-shaped; e. g., B. chauvei. 4. _Capitate_ (Fig. 91, e): Rods with a spherical enlargement at one pole; drumstick-shaped; e. g., B. tetani. The endo-spores remain within the parent cell for a variable time (in one case it is stated that germination of the spore occurs within the interior of the parent cell--"endo-germination"), but are eventually set free, as a result of the swelling up and solution of the cell membrane of the parent bacillus in the surrounding liquid, or of the rupture of that membrane. They then present the following characteristics: 1. Well-formed, dense cell membranes, which renders them extremely difficult to stain, but when once stained equally difficult to decolourise. 2. High refractility, which distinguished them from vacuoles.