STUDIES ON GERM CELLS 379 



Embryological investigation has gradually progressed from the 

 study of germ layers back to the study of the segmentation of 

 the egg, and from this to the organization of the ovum, and from 

 here to the genesis and localization of organ forming substances 

 in the oocyte. 



In the following pages the results of some investigations made 

 by the writer are described, and a discussion of the results ob- 

 tained by other investigators is given, in an attempt to deter- 

 mine the origin, nature, and significance of the Keimbahn- 

 determinants. 



I. THE HISTORY OF THE GERM CELLS IN INSECTS WITH SPECIAL 

 REFERENCE TO THE KEIMBAHN-DETERMINANTS 



1. INTRODUCTION 



The Keimbahn in animals was first described by Metschnikoff 

 ('65, '66) in the paedogenetic larvae of the fly, Miastor. Since 

 that time various investigators have been able to trace the germ 

 cells in many other species of insects, belonging to several differ- 

 ent orders, from early cleavage stages to the definitive germ 

 glands, and have discovered that a complete Keimbahn can also 

 be demonstrated in species belonging to other classes and phyla, 

 notably the Crustacea, the Nematoda, and the Chaetognatha. 

 The writer ('09) has published an account of our knowledge of 

 the origin and early development of the germ cells in insects up 

 to the year 1908, but no complete account of the Keimbahn in 

 other groups of animals has ever appeared. 



The data regarding this phase of the germ cell cycle are widely 

 scattered in the literature; frequently buried in treatises on 

 general embryology, and less often contained in contributions 

 devoted to this subject alone. The accounts found in current 

 reference books and text books are for the most part obsolete or 

 inaccurate. In the following account statements, with figures, 

 of the more important discoveries of other investigators have 

 been included in order to allow a general consideration of our 

 entire knowledge regarding the Keimbahn-determinants. 



