vi PREFACE TO THE FIRST EDITION. 



With the view of inducing independent thought on 

 the part of the student, and of diminishing the fre- 

 quency of that oft-heard query, "What shall I do 

 next?" experiments have been suggested wherever it is 

 possible. These have been arranged to illustrate the 

 salient points of the work and to attract attention to 

 the minute details, upon the observation of which so 

 much in bacteriology depends. 



PHILADELPHIA, December, 1891. 



