212 GERM-CELL CYCLE IN ANIMALS 



parts of the cytoplasm by some investigators ; in 

 some species they are extra-cellular bodies, such as 

 nurse cells. 



The accompanying table indicates the number and 

 diversity of the animals in which keimbahn-determi- 

 nants have been described, and shows the increasing 

 interest that has been given to this subject within re- 

 cent years, over half of the papers listed having been 

 published since 1908. Several cases have been re- 

 ferred to in the text, but omitted from the table be- 

 cause of insufficient evidence regarding their connec- 

 tion with the primordial germ cells. The list as 

 given includes representatives of the Ccelenterata, 

 Ch^tognatha, Nematoda, Arthropoda, and Ver- 

 TEBRATA. The tcrms applied to the various sub- 

 stances have been chosen evidently because of their 

 genesis, position in the egg, or supposed function. 



Table of Principal Cases of Visible Substances con- 

 cerned IN Differentiation of Germ Cells (in Chron- 

 ological Order) 



