INTRODUCTION. 19 



arteries and veins through intervening capillaries, thus 

 affording ocular proof of Harvey's discovery of the circu- 

 lation 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 contro- 

 versy 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 these minute organisms to exist in 

 such numbers that putrescent infusions, both animal and 

 vegetable, literally teemed with them, one drop of such 

 a liquid furnishing a banquet for millions. 



Abbe Lazzaro Spallanzani (1777) filled flasks with 

 organic infusions, 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. Spallan- 

 zani' 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 the objection aside by filling a flask 

 only half full of distilled water, to which animal and 

 vegetable matters were added, boiling the contents to 

 destroy the vitality of any organisms which might al- 

 ready exist in them, then sucking daily into the flask a 

 certain amount of air which had 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 with- 

 out the development of any infusorial life. 



It must have been a remarkably germ-free atmosphere 

 in which Schultze worked, for, as was shown by those 



