194 PRACTICAL BACTERIOLOGY. 



incubator, which is kept at a temperature of 20°. The 

 bacteria rapidly grow into threads, but only com- 

 mence to form spores after they have exhausted the 

 nourishment, a condition of things which arises in a 

 longer or shorter time, according to the amount of 

 agar-agar and the number of bacilli present. 



But spores can only be formed if a suflBcient quantity 

 of air has been enclosed in the cavity of the slide, as 

 they require an ample supply of oxygen in order to 

 develop. 



If necessary, the cover-glass may be raised with a 

 needle, in order to allow fresh air to enter, but it is 

 better at the first to choose a slide with a very large 

 cavity or one which has a glass cell cemented on to it. 



The first indication of the commencement of the 

 formation of spores is, that the threads become less 

 distinct and lose their great refractive power, whilst 

 fine granules make their appearance inside them. 

 Sometimes the cell walls appear to stand out more 

 clearly, but frequently they remain invisible. In each 

 cell a small bright point appears, which gradually 

 increases in size, whilst becoming brighter and more 

 refractive, so that it appears to shine out from the 

 faintly grey thread. Finally, it reaches the cell wall, 

 when it grows oval in shape, and at the same time 

 becomes clearly marked off by a sharp outline from 

 the surrounding protoplasm. As this body becomes 

 brighter and brighter the rodlet on the other hand 



