vi PREFACE 



the problems involved can be considered in a work 

 of this size, and those for which space can be found 

 must be limited in their treatment. For this reason 

 some periods in the germ-cell cycle are only briefly 

 mentioned, whereas others are more fully discussed. 

 The latter are naturally those in which the writer 

 is most interested and with which he is best ac- 

 quainted. Furthermore, the attempt is made to 

 present the data available in such a way as to make 

 it intelligible to those who have not been able to 

 follow in detail the progress of cytology during the 

 past few years. This can only be accomplished by 

 introducing many facts that are well known to 

 cytologists and zoologists in general, but are neces- 

 sary for the presentation of a complete account of 

 the subject. 



Much of the recent cytological work done on germ 

 cells has emphasized the events which take place 

 during the maturation of the eggs and spermatozoa, 

 that is, the periods of oogenesis and spermatogenesis. 

 These are, of course, very important phases of the 

 germ-cell cycle, but they should not be allowed to 

 overshadow the rest of the history of the germ cells. 

 Contrary to the usual custom, the period that is 

 emphasized in this book is not the maturation of 

 the germ cells, but the segregation of the germ cells 

 in the developing egg and the visible substances 

 (keimbahn-determinants) concerned in this process. 



It has been impossible to include in this book as 

 much illustrative material as desirable, but the bib- 

 liography appended indicates what data exist and 



