4 GENERAL MORPHOLOGY AND BIOLOGY 



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 

 protoplasm 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 individual after twenty-four hours 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. Sometimes division proceeds so 

 rapidly that the young individuals do not reach the adult size 

 before multiplication again occurs. This may give rise to 

 anomalous appearances. When bacteria are placed in unfavour- 

 able 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 individuals may be seen. The regularity in 

 structure and size is quite lost. The appearance of the protoplasm 

 also is often altered. Instead of, as formerly, staining well, it 

 does not stain readily, and may have a uniformly pale, homo- 

 geneous appearance, 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 globules which may be of large size. Such aberrant 

 and degenerate appearances are referred to as involution forms. 

 That these forms really betoken degenerative changes is shown 

 by the fact that, on their being again transferred to favourable 

 conditions, only slight growth at first takes place. Many 

 individuals have undoubtedly died, and the remainder which 



