vjjj PREFACE 



Because of their fundamental importance in connection with the problems 

 confronting the geneticist, the phenomena of nuclear division, chromo- 

 some reduction, and fertilization are described with considerable fullness, 

 and their relation to the problems of heredity is taken up in five special 

 chapters. With regard to many of the subjects treated, it has not been 

 found possible to formulate final conclusions, since in many cases nothing 

 more than tentative general statements are warranted by the facts in 

 our possession. In some chapters little more than catalogs of conflicting 

 opinions can be given, but in such a form the state of certain questions 

 is not inaccurately represented. The student entering upon the field of 

 cytology will be impressed by the large number of special points which 

 remain undetermined and general questions which await adequate 

 answers. If he can look upon cytology as a developing science, and if 

 he has reached the stage at which he no longer demands categorical 

 answers to all his questions, this book will be of interest to him as much 

 for the problems it raises as for those it helps to solve. Not the least of 

 its functions is to indicate lines of research along which he can hope to 

 make contributions to the subject. 



In compiling his materials the author has not hesitated to draw very 

 freely upon the writings of others. In many, cases where direct quotation 

 is not made, the language of the originals has been closely followed in 

 order to lessen the liklihood of misrepresentation. His great debt to 

 Professor Wilson's book will be apparent to all those familiar with that 

 admirable work. The majority of the diagrams and a number of the 

 other figures are new. Most of the latter, however, have been redrawn 

 from works cited in the text, not only that the value of the book may be 

 enhanced by the presence of authoritative illustrations, but also that the 

 student may be encouraged to become more familiar with the original 

 papers. The general systematic positions of organisms indicated in the 

 text by their scientific names only may be ascertained by referring to 

 the generic names in the index. 



The illustrations are largely the work of Miss Mildred Stratton, in 

 whose skill and spirit of cooperation the author has had invaluable 

 assistance. The criticisms of the text kindly given by Professor C. J. 

 Chamberlain of the University of Chicago and Professor R. A. Emerson of 

 Cornell University have been very highly appreciated. Acknowledge- 

 ments are also made to the author's other colleagues for their advice and 

 continued encouragement. Further criticisms looking toward the im- 

 provement of future editions will be welcomed. 



ITHACA, NEW YORK, 

 September 8, 1920. 



