84 STUDY OF BACTERIA 



swiftly that the eye can hardly follow them, or they may merely 

 roll or waddle across the field slowly. Direct division, if proceeding 

 under the best conditions, requires but 15 to 40 minutes. It is best 

 observed in a warm stage or when working in a room kept at a 

 temperature of 35 C. Sporulation occurs differently in different 

 species. In some it will be found soon after the culture has been 

 removed from the incubator, while in others several hours are 

 required. Sporulation, it must be remembered, is a resistant 

 stage when unfavorable conditions are met. 



The Gruber-Widal reaction is thus studied. A drop of the 

 serum and bouillon culture, mixed in proper proportions, is dropped 

 on a cover-slip, which is then placed, drop downwards, over the 

 cavity of the slide (hanging drop, fig. 21). (See Agglutination.) 



Staining bacteria is a matter that is easily accomplished, and 

 very many staining solutions and methods have been invented for 

 this purpose. 



The simplest procedure is to take a drop of pus, blood or culture, 

 and spread it upon a very clean slide with a sterilized platinum 



FIG. 21. Hanging drop, over hollow ground slide. (Williams.) 



needle. The matter must be spread thinly and evenly. After the 

 water has evaporated and the preparation has become dry without 

 the use of heat, it must be fixed. To do this various agents are used. 

 The object of the fixing is to coagulate the protoplasm of the cells, 

 and to fasten all the smeared matter fast to the glass, so that the 

 staining fluid and water will not wash them off. This is accom- 

 plished, in the case of a slide, by holding it in the apex of a bunsen 

 flame until quite warm to the hand. Great care must be used not to 

 char the film. Experience is needed to fix slide smears correctly. 

 The beginner would do well to use cover-slips. If a cover-slip is 

 used it must be passed through the flame three times rapidly. After 



