4 GENERAL MORPHOLOGY AND BIOLOGY 



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 places 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. 



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. 137). 

 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. cholerse). 



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 individuals may be seen. The regularity in structure 



