1 8 Introduction 



"Watching meat in its passage from freshness to decay, prior to 

 the appearance of maggots, he invariably observed flies buzzing 

 around the meat and frequently alighting on it. The maggots, he 

 thought, might be the half-developed progeny of these flies. Placing 

 fresh meat in a jar covered with paper, he found that although the 

 meat putrefied in the ordinary way, it never bred maggots, while 

 meat in open jars soon swarmed with them. For the paper he 

 substituted fine wire gauze, through which the odor of the meat 

 could rise. Over it the flies buzzed, and on it they laid their eggs, 

 but the meshes being too small to permit the eggs to fall through, 

 no maggots generated in the meat; they were, on the contrary, 

 hatched on the gaUze. By a series of such experiments Redi 

 destroyed the belief in the spontaneous generation of maggots in 

 meat, and with it many related beliefs." 



In 1683 Anthony van Leeuwenhoek, justly called the "Father 

 of microscopy," demonstrated the continuity of arteries and veins 

 through intervening capillaries, thus affording ocular proof of 

 Harvey's discovery of the circulation of the blood; discovered 

 bacteria, seeing them first in saliva, discovered the rotifers, and first 

 saw the little globules in yeast which Latour and Schwann subse- 

 quently proved to be plants. 



Leeuwenhoek involuntarily reopened the old controversy about 

 spontaneous generation by bringing forward a new world, peopled 

 by creatures of such extreme minuteness as to suggest not only a 

 close relationship to the ultimate molecules of matter, but an easy 

 transition from them. 



In succeeding years the development of the compound microscope 

 showed that putrescent infusions, both animal and vegetable, 

 teemed with minute living organisms. 



Abbe Lazzaro Spallanzani (1777) filled flasks with organic in- 

 fusions, sealed their necks, and, after subjecting their contents to 

 the temperature of boiling water, placed them under conditions 

 favorable for the development of life, without, however, being able 

 to produce it. Spallanzani's critics, however, objected to his 

 experiment on the ground that air is essential to life, and that in 

 his flasks the air was excluded by the hermetically sealed necks. 



Schulze (1836) set this objection aside by filling a flask only half 

 fuU of distilled water, to which animal and vegetable matters were 

 added, boiling the contents to destroy the vitality of any organisms 

 which might already exist in them, then sucking daily into the 

 flask a certain amount of air which was passed through a series 

 of bulbs containing concentrated sulphuric acid, in which it was 

 supposed that whatever germs of life the air might contain would 

 be destroyed. This flask was kept from May to August; air was 

 passed through it daily, yet without the development of any 

 infusorial life. 



It must have been a remarkably germ-free atmosphere in which 



