INTRODUCTION. 3 



and that it consists of a single cell, not of an aggregation of cells, 

 as do most of the common plants and animals. 



Under favorable conditions bacteria undergo rapid multipli- 

 cation. The individuals in some forms divide into two cells, 

 in other forms into four cells, in others again into eight cells 

 simultaneously. The process takes place by direct cell division, 

 and is called fission. 



Under certain conditions, bright, glistening bodies make their 

 appearance in certain bacteria, and become larger and larger, 

 while the cells in which they develop break up into fine frag- 

 ments. These bodies are called spores, and represent a resting 

 stage in some respects resembling the seeds of Jiigher plants. 

 They have much greater resisting power against injurious 

 influences than is possessed by the growing or vegetative forms. 

 There are spores that can withstand boiling for hours, but 

 fortunately that is not true, as far as we know, of the spores of 

 any of the bacteria that produce disease. The earlier inves- 

 tigators observed the appearance of bacteria in nutrient infu- 

 sions which they had endeavored to sterilize by heat. They 

 looked upon this fact as indicating the possibility Of spontaneous 

 generation, and it furnished the chief support of that theory. 

 Probably their infusions contained very resistant spores, and 

 were in reality not sterile. 



From these facts a definition for bacteria may be formulated. 



Bacteria (Greek ^axTrjpwv, meaning a little stick) are ex- 

 tremely minute, unicellular plants, which have no chlorophyll, 

 and which divide by fission. They are sometimes called schiz- 

 omycetes. In every-day language they are known as microbes, 

 and also as germs. They are generally classed with the fungi. 

 In some respects they seem quite closely related to the algae or 

 simplest green plants, and, on the other hand, they have strong 

 points of likeness with some of the unicellular animals belong- 

 ing to the infusoria. 



Bacteria are divided'into two great groups : 



