PREFACE 
Tuts 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 have 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 
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