MULTIPLICATION OF BACTERIA. 5 



species of cocci. They 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. It is doubtful whether, as in many algae, they 

 are prolongations of the protoplasm through the envelope, 

 or whether they are merely appendages of the latter, in 

 which case some have been inclined to doubt whether they 

 are really organs of locomotion or not. For the present 

 purpose it is sufficient to state that while flagella have been 

 observed in all the lower bacteria which are motile, except 

 perhaps in some special spiral forms, and not in the non- 

 motile forms, some motile forms of the higher bacteria 

 are known in which motility is not associated with the 

 possession of special organs, but is probably due to contrac- 

 tility of the protoplasm itself. 



Multiplication. When a bacterial cell is placed in 

 favourable surroundings it multiplies ; as has been said, this, 

 in the great majority of cases, takes place by simple fission. 

 In this process a constriction appears in the middle 

 and a transverse unstained line develops across the proto- 

 plasm at that point. The process goes on till two indi- 

 viduals can be recognised, which may remain for a time 

 attached to one another, or become separate, according to 

 the character of the envelope, as already explained. In 

 most bacteria growth and multiplication go on with great 

 rapidity. A bacterium may reach maturity and divide in 

 from twenty minutes to half an hour. In the latter case a 

 simple calculation will show that, at the end of twenty-four 

 hours, from one individual 17,000,000 similar individuals 

 will be produced. As shown by the results of artificial 

 cultivation, others, such as the tubercle bacillus, multiply* 

 much more slowly. When bacteria are placed in unfavour- 

 able conditions as regards food, etc., growth and multipli- 

 cation take place with difficulty. In the great majority of 

 cases this is evidenced by changes in the appearance of the 

 protoplasm. Instead of its maintaining the regularity of 

 shape seen in healthy bacteria, various aberrant appearances 

 are presented. This occurs especially in the rod -shaped 



