84 BACTERI 



this multiplied by 4,000 gives 72,000 cm. = 720 metres = 2362 

 feet. We therefore get the proportion as 2362 feet, or 

 nearly six times the height of St. Paul's, is to the height of 

 the present volume, so the length of Fig. 15 is to that of 

 Bacterium termo. 



It was mentioned above that at a certain stage of putre- 

 faction a scum forms on the surface of the fluid. This film 

 consists of innumerable motionless Bacteria imbedded 

 in a transparent gelatinous substance formed of mycoprotein 

 (Fig. 14, B). After continuing in the active condition for a 

 time the Bacteria rise to the surface, lose their flagella, and 

 throw out this gelatinous substance in which they lie imbedded. 

 The bacterial jelly thus formed is called a zooglcea. Thus 

 in Bacterium termo, as in so many of the organisms we have 

 studied, there is an alternation of an active with a resting 

 condition. 



During the earlier stages of putrefaction Bacterium termo 

 is usually the only organism found in the fluid, but later on 

 other microbes make their appearance. Of these the com- 

 monest are distinguished by the generic names Micrococcus, 

 Bacillus, Vibrio, and Spirillum. 



Micrococcus (Fig. 16) is a minute form, the cells of which 

 are about 2/x (5-5-^ mm.) in diameter. It differs from 

 Bacterium in being globular instead of spindle-shaped and 

 in having no motile phase. Like Bacterium it assumes the 

 zooglaea condition (Fig. 16, 4). 



Bacillus is commonly found in putrescent infusions in 

 which the process of decay has gone on for some days : as 

 its numbers increase those of Bacterium termo diminish, 

 until Bacillus becomes the dominant form. Its cells (Fig. 

 17) are rod-shaped and about 6/x ( T iy- mm.) in length in the 



