8 GENERAL MORPHOLOGY AND BIOLOGY 



life again occurs, it is from them that multiplication is said to take place. 

 From the fact that there is no new formation within the protoplasm, 

 but that it is the whole of the latter which participates in the change, 

 these individuals have been called arthrospores. The existence of such 

 special individuals amongst the lower bacteria is extremely problematical. 

 They have no distinct capsule, and they present no special staining 

 reactions, nor any microscopic features by which they can be certainly 

 recognised, while their alleged increased powers of resistance are very 

 doubtful. All the phenomena noted can be explained by the undoubted 

 fact that in an ordinary growth there is very great variation among 

 the individual organisms in their powers of resistance to external 

 conditions. 



Motility. — As has been stated, many bacteria are motile. 

 Motility can be studied by means of hanging-drop preparations 

 (vide p. 69). The movements are of a darting, rolling, or 

 vibratile character. The degree of motility depends on the 

 species, the temperature, the age of the growth, and on the 

 medium in which the bacteria are growing. Sometimes the 

 movements are most active just after the cell has multiplied, 

 sometimes it goes on all' through the life of the bacterium, 

 sometimes it ceases when sporulation is about to occur. Motility 

 is associated with the possession of fine wavy thread-like 

 appendages called flagella, which for their demonstration require 

 the application of special staining methods (vide Fig. 1, q; 

 and Fig. 107). They have been shown to occur in many bacilli 

 and spirilla, but only in a few species of cocci. They vary in 

 length, but may be several times the length of the bacterium, 

 and may be at one or both extremities or all round. When 

 terminal they may occur singly or there may be several. Some- 

 times complicated spiral tresses of free flagella are found in 

 bacterial cultures ; the development of these is difficult to 

 explain. The nature of flagella has been much disputed. Some 

 have held that, unlike what occurs in many algae, they are not 

 actual prolongations of the bacterial protoplasm, but merely ap- 

 pendages of the envelope, and have doubted whether they are 

 really organs of locomotion. There is now, however, little doubt 

 that they belong to the protoplasm. By appropriate means the 

 central parts of the latter can be made to shrink away from the 

 peripheral (vide infra, " plasmolysis "). In such a case movement 

 goes on as before, and in stained preparations the flagella can be 

 seen to be attached to the peripheral zone. It is to be noted 

 that flagella have never been demonstrated in non-motile bacteria, 

 while, on the other hand, they have been observed in nearly all 

 motile forms. There is little doubt, however, that all cases of 

 motility among the bacteria are not dependent on the possession 

 of flagella, for in some of the special spiral forms, and in most 



