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, usually 

 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. 

 St 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 b'acillus, 

 multiply much more slowly. In some cases the bacterial cell 

 enlarges before division, in others the cell divides and each 

 element then expands to its adult size. If, in the latter alterna- 

 tive, multiplication is proceeding rapidly great variation in the 

 size of the individuals may be observed, and this may give rise to 

 anomalous appearances. 



From investigations by Graham-Smith and others, it appears that 

 the consistence of the envelope may have an importance in modifying 

 the naked-eye and low-power appearances presented by bacterial colonies 

 which constitute a feature in the identification of species (see p. 139). 

 Graham-Smith, working with bacilli, differentiates four groups a " loop- 

 forming," in which the envelope is so tough that, after division, rupture 

 * but rarely occurs (b. anthracis) ; a " folding " group, in which the envelope 

 is so flexible and extensile that the members of a chain can be folded on 

 one another as successive divisions take place (b. pestis) ; a "snapping" 

 group, in which partial rupture of the envelope occurs on division 

 (b. diphtherias) ; and a "slipping" group, where the envelope readily 

 breaks, and successively developed bacilli slip past each other (v. cholene). 



When bacteria are placed inma/fcourable 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 appearances 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- 



