PREFACE ix 



investigators, I must make a virtue of necessity. Many of the facts 

 could not possibly have been illustrated by new figures equal in value 

 to those of special workers in the various branches of cytological 

 research, Qjven had the necessary material and time been available. 

 But, apart from this, modern cytology extends over so much debatable 

 ground that no general work of permanent value can be written that 

 does not aim at an objective historical treatment of the subject; and 

 I believe that to this end the results of investigators should as far as 

 practicable be set forth by means of their original figures. Those 

 for which no acknowledgment is made are original or taken from 

 my own earlier papers. 



The arrangement of the literature lists is as follows. A general 

 list of all the works referred to in the text is given at the end of the 

 book (p. 343). These are arranged in alphabetical order, and are 

 referred to in the text by name and date, according to Mark's con- 

 venient system. In order, however, to indicate to students the more 

 important references and partially to classify them, a short separate 

 list is given at the end of each chapter. The chapter-lists include 

 only a few selections from the general list, comprising especially 

 works of a general character and those in which reviews of the 

 special literature may be found. 



E. B. W. 



COLUMBIA UNIVERSITY, NEW YORK, 

 July, 1896. 



