HISTORIC DEVELOPMENT OF IMMUNOLOGY 23 



identified today as belonging to one or another 

 of the chief group forms. 



Up to the period of Pasteur's investigations, 

 the role played by bacteria in various familiar 

 natural processes such as putrefaction, decay 

 and fermentation had been, perhaps, vaguely 

 suspected but had not received conclusive dem- 

 onstration. The memorable researches of Pas- 

 teur upon spontaneous generation and fermen- 

 tation imparted to the study of bacteria biologic 

 importance that it had not heretofore possessed. 

 Bacteria and kindred microorganisms were 

 shown to be responsible for setting in motion 

 and carrying out many everyday processes, the 

 nature of which had not before been understood. 

 It was almost entirely through the work of Pas- 

 teur that bacteria emerged from the relative ob- 

 scurity as microorganisms chiefly of interest to 

 the professional biologist, and took a conspicu- 

 ous position in natural science as a group of 

 microorganisms whose activities were full of a 

 far-reaching significance for mankind. 



BACTERIA 



Bacteria may be defined as extremely minute, 

 unicellular microorganisms which reproduce 



