8 GENERAL MORPHOLOGY AND BIOLOGY. 



power of resistance than the other forms ; further, when vegetative 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 condition^. 



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

 Motility can be studied by means of hanging-drop preparations 

 (vide p. 68). The movements are of a darting, rolling, or 

 vibratile character. The degree of motiUty depends on the 

 temperature, on the age of the growth, and on the medium in 

 which the bacteria are. 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. i. No. 12; and Fig. 115). 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. The nature of these 

 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 appendages of the en- 

 velope, and have doubted whether they are really organs of 

 locomotion. There is now, however, little doubt that they be- 

 long 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 



