•STUDIES ON GERM CELLS 377 



others) or indefinite number (so far as we know) of oogonia 

 or spermatogonia; 



5. In some cases the differentiation of oogonia into nurse cells 

 and ultimate oogonia, and the spermatogonia into Sertoli cells 

 and ultimate spermatogonia; 



6. The growth of the ultimate oogonia and spermatogonia to 

 form primary oocytes and primary spermatocytes; 



7. Maturation; 



8. Fertilization (if not parthenogenetic) . 



This list of periods differs from the series usually recognized 

 in that it starts with the beginning of the germ cell cycle instead 

 of at a comparatively late stage, i.e., with oogonia and sper- 

 matogonia. Certain of these periods, especially those of matura- 

 tion and fertilization, have been emphasized by investigators 

 much more than others. Many of the fundamental problems 

 of heredity and development are, however, concerned with the 

 events which take place during the less known stages. 



For a number of years the writer has been particularly in- 

 terested, in the segregation of germ cells during embryonic devel- 

 opment, and has studied especially certain visible substances 

 which are present in the egg before cleavage begins, and later 

 become part of the material contained in the primordial germ 

 cells. In the eggs of certain Chrysomelid beetles this substance 

 was termed the 'pole-disc' (fig. 7, A, g.c.d., p. 403), and the gran- 

 ules of which the pole-disc is composed were called 'germ-cell 

 determinants' because they enable us to determine which cells 

 will become germ cells. Since this term is Ukely to be misin- 

 terpreted, the granules of the pole-disc and other similar sub- 

 stances that have been found in the eggs of animals are, in this 

 paper, called 'Keimbahn-determinants,' since they furnish the 

 means of recognizing the germ-cell material in the undivided 

 egg. or in cleavage stages, and thus make it possible for us to 

 determine the 'Keimbahn' from one generation to another. 



The following events may be listed in the history of the Keim- 

 bahn-determinants : 



1. Localization of the Keimbahn-determinants in the oocyte 

 or mature egg; 



