vi PREFACE 



ration of various portions of the text. The author desires to 

 express his especial indebtedness to his good friend, Mr. H. O. 

 Hanson, for the preparation of many of the illustrations; to Mr. 

 W. A. Andrews, for microscopical studies from which drawings 

 have been made, and to Miss Mary L. Williams, for the execu- 

 tion of many of the studies. Special mention is also made of the 

 author's indebtedness to the assistance and collaboration of his 

 former pupils and present associates, Drs. E. Altenburg and H. 

 J. Muller. 



CARLTON C. CURTIS. 

 COLUMBIA UNIVERSITY, 

 June. 1918. 



