PREFACE 



This book is the result of a course of lectures 

 delivered during the past school year before a class 

 in Cellular Biology at the University of Michigan. 

 Many of the most important recent additions to 

 our knowledge of heredity have resulted from the 

 study of the germ cells, especially those of animals. 

 This study is now recognized as one of the chief 

 methods of attacking certain problems in genetics 

 and must be employed in correlation with animal 

 breeding before we can hope to obtain an adequate 

 explanation of the results of hybridization. For- 

 tunately the cytological studies of the germ cells, 

 both observational and experimental, have kept 

 pace with the rapid advances in our knowledge of 

 plant and animal breeding which h^ve been made 

 since the rediscovery of Mendel's investigations in 

 1900. The term " Germ-Cell Cycle " is meant to 

 include all those phenomena concerned with the ori- 

 gin and history of the germ cells from one genera- 

 tion to the next generation. The writer has, with 

 few exceptions, limited himself to a consideration of 

 the germ cells in animals because the cycle is here 

 more definite and better known than in plants. 



It is obvious to any one familiar with this subject 

 that only a few of the many interesting phases of 



