20 BACTERIA : FORMS AND REPRODUCTION 



is dependent upon temperature, air supply, nutrition, 

 and other circumstances, but under favourable conditions 

 division may occur once in every hour or less, so that 

 from one individual many millions of descendants can 

 be produced in a day when placed in milk or other 

 suitable nutritive liquids. 



(ii) Reproduction by Spores. Under certain con- 

 ditions, and especially those which lead to unfavourable 

 or unhealthy growth, such as unsuitable temperature, 

 dryness, excess or deficiency of air supply, exhaustion 

 of nutrient materials and accumulation of waste pro- 

 ducts, the vegetative division of bacteria is retarded and 

 stopped. The protoplasm within the cell contracts and 

 forms an oval body known as a spore or endospore (Fig. 

 1 1 ), which, on account of its high refraction, appears as a 

 bright glistening spot surrounded by an apparently empty 

 shell of the mother-cell. The latter sooner or later 

 swells up and dissolves, leaving the free spore behind. 

 The protoplasm of the spore is denser than that of the 

 original mother-cell, and the coat of the spore is a some- 

 what thick membrane which is not readily permeable to 

 dyes or other solutions. The number of spores which 

 originate in each bacterium is generally one, though 

 in one or two kinds two or more spores are seen in 

 each cell. The spore occupies various positions in the 

 cell arising in some species at one end, while at others 

 it is formed near the middle of the bacterium. In the 

 genus, Clostridium, the mother-cell assumes a peculiar 

 spindle-shaped form, swollen in the middle and narrow 

 at the ends when spore formation takes place : it may also 

 be shaped like a drumstick as in the bacilli which cause 

 tetanus (Fig. 1 1). In some species of bacteria endospores 

 are not found, but their vegetative cells cease to divide, be- 



