INTRODUCTION. 3 



not be seen as individuals without the aid of the microscope, 

 although great numbers of them taken together may form a 

 plainly visible mass of growth. When they are examined 

 with the microscope they appear as little, round, rod-shaped or 

 curved bodies, which may be likened to so many periods, dashes 

 and commas. It is at once perceived that each bacterium is an 

 individual by itself, 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 divi- 

 sion, 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 

 fragments. These bodies are called spores, and represent a 

 resting stage in some respects resembling the seeds of higher 

 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 it not true of the spores of any of 

 the bacteria that produce disease, as far as is yet known. The 

 earlier investigators observed the appearance of bacteria in 

 nutrient infusions 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. 



Definition. — ^From these facts a definition for bacteria may be 

 formulated. 



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

 tremely minute, unicellular plants, which have no chlorophyll, and 



