10 APPLIED BACTERIOLOGY 



unfavourable conditions. Spore formation may take place 

 in two ways : firstly, by ' endogenous spores ' (internal 

 spores) ; secondly, by ' arthrospores.' 



(a) Endogenous Spores. — When the formation of the spores 

 takes place in the mother-cell, the protoplasm is seen to 

 contract, giving rise to one or more highly refracting bodies, 

 which are the spores. The enclosing membrane of the 

 organism then breaks away, leaving the spores free. 



(b) Arthrospores. — When the spore is not formed in the 

 parent bacillus, but when entire cells (owing to lack of 

 favourable conditions of growth) become converted into 

 spores, the formation is known as ' arthrogenous,' the 

 single individuals being called ' arthrospores.' 



When the conditions are again favourable, these spores 

 germinate, giving rise to new bacilli. The germinating 

 spore becomes elongated, and loses its bright appearance ; 

 the outer membrane becomes ruptured, and the young 

 bacillus is set free. Certain conditions, such as the presence 

 of oxygen in the case of the anthrax bacillus, give rise to 

 the formation of spores ; while various kinds of bacteria 

 secure their existence by developing spores when there is 

 lack of proper food material. 



With reference to the incredible rapidity with which the 

 bacteria multiply under conditions favourable to their 

 growth and development, Cohn writes as follows : ' Let 

 us assume that a microbe divides into two within an hour, 

 then again into eight in the third hour, and so on. The 

 number of microbes thus produced in twenty-four hours 

 would exceed sixteen and a half millions ; in two days 

 they would increase to forty-seven trillions ; and in a week 

 the number expressing them would be made up of fifty-one 

 figures. At the end of twenty-four hours the microbes 

 descended from a single individual would occupy ^^5- of a 

 hollow cube, with edges -oV of an inch long, but at the end 



