PREFACE 



populations of cells, whose organized individually is a result, not a 

 cause, of the differentiation of those cells. 



These researches have been supported by grants from the Grad- 

 uate School of the University of Minnesota, for which I am deeply 

 grateful. I wish also to express here my gratitude to the Depart- 

 ment of Dairy Industry of Cornell University, which has provided 

 stenographic assistance in the preparation of the manuscript; to Dr. 

 W. P. Larson, head of the Department of Bacteriology at the Univer- 

 sity of Minnesota, who has made every effort to provide the special 

 equipment necessary for my work; to Dr. J. M. Sherman, who has 

 kindly read the manuscript of Chapter I; and to Dr. Otto Rahn, 

 who has similarly read Chapter II for me. Above all I must ac- 

 knowledge my obligation to Dr. R. E. Scammon, of the Department 

 of Anatomy of the University of Minnesota, without whose sugges- 

 tions this work would not have been undertaken, and without whose 

 kindly assistance and friendly criticism it could not have been carried 

 through. 



I am indebted to the publishers of the Journal of Infectious Dis- 

 eases, the Journal of Experimental Zoology, the Journal of Experi- 

 mental Medicine, the Journal of Hygiene, the Proceedings of the 

 Society for Experimental Biology and Medicine, and of Pearl's 

 "Biology of Population Growth" for permission to reproduce certain 

 of the illustrations. Due acknowledgement will be found in the 

 legends accompanying the various figures used. 



XI 



