and Laboratory Methods. 



2271 



ferred to three " form types,'' the sphere, the straight rod, the bent rod. These 

 fundamental form types are shown in Fig. 8. The similarity of form among 

 bacteria, which in reality may be very different, can be imagined when it is 

 understood that into the above mentioned form-types fall at least 1000 fairly well 

 defined bacterial species. The inadequacy of the mere microscopical examina- 

 tion of bacteria will be further realized by the fact that it is absolutely impossi- 

 ble to distinguish, microscopically, between bacteria which are perfectly harmless 

 and those which are most deadly in their action on man. It will, therefore, be 

 quite apparent that in studying the bacteria we cannot rely simply on the micro- 

 scopical examination of them. For while it is true that we might learn much 

 about the form and structure of bacteria, be able to recognize a number of differ- 

 ent species, work out something of their distribution, and perhaps divine 

 something of the relationship which they bear to disease, it is still true that the 

 science of bacteriology dates from what are now known as culture methods. 



Fig. 9. — Method of Reproduction among Bacteria. The upper series 

 represents the fission of the spherical forms, and the lower that 

 of the rod forms. 



By means of cultures we are able to rear the bacteria in large numbers and 

 study, not the single individual, but aggregates of similar bacteria which usually 

 form masses visible to the naked eye, and frequently these masses or " colonies" 

 as they are called are of considerable size. 



2'' he rate of mnltiplicafion among Bacteria. A single generation, which is to 

 be considered as the time elapsing between two successive divisions, is frequently 

 only the fraction of an hour. In some species the individual may grow to 

 maturity and reproduce in twenty minutes. The rate of multiplication thus be- 

 comes enormous as time goes on. Some idea of this rate is obtained in the fol- 

 lowing table : 



