Besides the three typical cell forms bacteria frequently show very great irregularities in shape. They may be pointed, bulged, club-shaped or even slightly branched. These peculiar and bizarre forms practically always occur when some of the necessary conditions for normal growth, discussed in [Chapters VI] and [VII], are not fulfilled. They are best regarded as involution or degeneration forms for this reason ([Fig. 40]). In a very few cases it is not possible to obtain the organism without these forms (the diphtheria group). It is probable that these cell forms are normal in such cases, or else conditions suitable for the normal growth have not been obtained.
Fig. 37.—Spirilla.
Fig. 38.—Vibrio forms of spirilla. Compare with [Fig. 36].
Fig. 39.—Spirochetes.
Fig. 40.—Involution forms. The organisms are tapering and branched at one end.