PREFACE vii 



lop-sidedness of recent cytological work, but one of the aims 

 of the book is to indicate that many other problems of 

 equal interest remain to be investigated. 



In a book of this size it is impossible to mention a large 

 part of the facts, and many investigations of the first im- 

 portance are not referred to. The aim has been rather to 

 give representative examples, and the student who wishes 

 to pursue the matter further will have little difficulty in 

 finding the necessary references in the papers given in the 

 bibliography. This list includes few papers not referred to 

 in the text, but is intended to be sufficiently representative 

 to enable the student to find the other literature that he 

 may need. Although much has been omitted, the attempt 

 to state the facts that are mentioned in as small compass as 

 possible has necessitated rather serious condensation, and 

 the writer is painfully aware that some parts of the book do 

 not offer such easy reading as they might have done had he 

 allowed himself more space. 



In choosing figures for the illustrations, an attempt has 

 been made to employ as far as possible copies of original 

 figures, and to make use of diagrams only when they seem 

 necessary for a clear comprehension of the subject. Works 

 on cytology in the past have seemed to the writer to err in 

 the too frequent employment of diagrams, which sometimes 

 seem to the novice to bear no very close relation to the 

 actual figures that he sees with the microscope, and though 

 the methods of reproduction employed have necessitated 

 a certain amount of simplification in some cases, it is hoped 

 that the use of figures from original papers may to some 

 extent avoid this defect. A few of the purely diagrammatic 

 figures are new; others are borrowed from well-known books 

 and articles. In every case the source of borrowed figures 

 is acknowledged, and the reference given in the literature 



