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 zodlogists 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 odgenesis 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 
