4 GENERAL MORPHOLOGY AND BIOLOGY. 



When this occurs, it is known as a zooglcea mass. On the other 

 hand, if the envelope has not this cohesive property the sepa- 

 ration of individuals may easily take place, especially in a fluid 

 medium in which they may float entirely free from one another. 

 Many of the higher bacteria possess a sheath which has a much 

 more definite structure than is found among the lower forms. 

 It resists external influences, possesses elasticity, and serves to 

 bind the elements of the organism together. 



Reproduction among the Lower Bacteria. — 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 the 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 individuals 

 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. If division takes place only every hour, from one indi- 

 vidual after twenty-four hours 1 7,000,000 similar individuals will 

 be produced. As shown by the results of artificial cultivation, 

 others, such as the tubercle bacillus, multiply much more slowly. 

 Sometimes division proceeds so rapidly that the young indi- 

 viduals do not reach the adult size before multiplication again 

 occurs. This may give rise to anomalous appearances. When 

 bacteria are placed in unfavourable conditions as regards food, 

 etc., growth and multiplication 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 varieties, where flask-shaped or dumb-bell-shaped indi- 

 viduals may be seen. The regularity in structure and size is 

 quite lost. The appearance of the protoplasm also is often 

 altered. Instead of, as formedy, staining well, it does not stain 

 readily, and may have a uniformly pale, homogeneous appear- 

 ance, while in an old culture only a small proportion of the 

 bacteria may stain at all. Sometimes, on the other hand, a 

 degenerated bacterium contains intensely stained granules or 



