GENERAL MORPHOLOGY AND PHYSIOLOGY 55 



When the bacteria approach the end of the life cycle, or when the 

 conditions for growth and septation become unfavorable, spore formation 

 may tsike place. However, not all species of bacteria form spores (endo- 

 spores). For example, most of the pathogens do not form spores. Each 

 cell forms one spore only, there being apparently no exception to this 

 rule. In most cases the spore occupies a position nearer one end of the 

 cell, more rarely it occurs in a median position. As to form the spore is 

 generally somewhat oval in the direction of the long axis of the cell. It 

 may be more or less irregular in outline, as in Bacillus botuUnus, the cause 

 of botulism. The cell which contains a median spore causing a bulging of 

 the cell is called a Clostridium. If it causes a terminal bulging it is called 





a 



<=^ 



Fig. 9. — Illustrating polymorphism or pleomorphism. a to d, inclusive, represent 

 different forms of the same organism — the Diphtheria bacillus. (See also Figs. 46-50 

 inclusive.) 



a plectridium, also drum stick bacillus (Tronmielschlager bacillus). The 

 spore is formed from the cell plasm, and differs from it in its higher re- 

 fractive index and its pecuUar resistance to the action of stains. As soon 

 as spore formation is complete, the rest of the cytoplasm dies, the cell- 

 wall disintegrates, and the spore is thus set free. Spores have a remark- 

 able resisting power to high temperatures and other unfavorable conditions. 

 In a dry atmosphere they may lie dormant for a long time, even several 

 years. Boihng from one to two hours does not kill some of them (spores 

 of hay bacillus). As soon as the spores are placed in suitable media 

 (adequate warmth, moisture, and food supply) they develop into new 

 individuals, which continue to septate until spore formation again takes 



place. 



The classification given above, into families and genera, and Figs. 2 to 

 10, inclusive, will serve to give a fairly good idea of the general structural 

 characteristics of microbes. 



