iv PREFACE TO THE TENTH EDITION 



Recently work upon that ill-defined group of micro- 

 organisms, the Spirochsetaceae, has at least begun to be 

 systematized, and already our knowledge of their nature, 

 their relation to disease and the possible distribution of this 

 group have been greatly extended. Sufficient of these 

 studies has been incorporated in this edition to give to the 

 student an idea of what has been accomplished and the 

 methods used. New illustrations accompany this section. 



Since nothing has transpired to shake the foundations of 

 the science of bacteriology, the fundamental features of the 

 book, except for minor changes, remain as in the last edition. 



A. C. A. 



UNIVERSITY OP PENNSYLVANIA, 

 1921. 



