vi PREFACE. 



especially as regards morphology, life cycle, methods 

 of differentiation, and relation to disease, and to in- 

 clude everything of value resulting from the inves- 

 tigation of these interesting and important parasites. 

 I have tried to be conservative in the conclusions 

 drawn from my own work and that of others, for 

 there are many questions still unsettled regarding 

 the status of some of the species of amoeba* described, 

 and it is more than probable that further research 

 will result in the elimination of several species which 

 are to-day held by some observers to be valid. 



I desire to here express my deepest gratitude to 

 Brigadier General George H. Torney, Surgeon 

 General of the United States Army, for constant 

 encouragement and assistance, and for opportunities 

 afforded me for research work, and to Dr. Simon 

 Flexner, Director of the Rockefeller Institute for 

 Medical Research, for many favors extended while 

 working in that Institute and for his interest in the 

 publication of this work. My thanks are also due 

 Major Frederick F. Russell, U. S. Army, Director 

 of the Bacteriological Laboratories of the Army Med- 

 ical School, and to those authors whose illustrations 

 are here reproduced, and from whose investigations I 

 have profited. Credit has been given in every 

 instance, I believe, but if not, pardon is asked for an 

 unintentional omission. 



CHARLES F. CRAIG. 



WASHINGTON, D. C., July 10, 1911. 



