KEIMBAHN- DETERMINANTS 231 



products become somatic cells." Here, however, 

 the pole-disc granules were considered the essential 

 substance. 



The appearance of the keimbahn-determinants at a 

 certain time and in a certain place, and their deter- 

 minate segregation, point unmistakably to an under- 

 lying regulating mechanism. These phenomena have 

 some definite relation to the fundamental organiza- 

 tion of the egg and require an investigation of our 

 present knowledge of this subject. 



The isotropism of the egg as postulated by Pfluger 

 and the "cell interaction" idea especially developed 

 by O. Hertwig and Driesch have given way before 

 the beautiful researches tending to uphold the hy- 

 pothesis of "germinal localization" proposed by His 

 and championed by so many investigators within 

 the past two decades. The starting point for embry- 

 ological studies has shifted from the germ layers 

 to the cleavage cells and from these to the undivided 

 egg. Organization, which Whitman (1893) main- 

 tains precedes cell-formation and regulates it, is now 

 traced back to very early stages in the germ-cell 

 cycle and held responsible for the cytoplasmic lo- 

 calization in the egg. 



One of the fundamental characteristics of the egg is 

 its polarity. It has been known for about thirty 

 years that the eggs of insects are definitely ori- 

 ented within the ovaries of the adults. Moreover, 

 gravity and the action of centrifugal force have no 

 effect upon the polarity of insect eggs (Hegner, 1909^) . 

 Giardina (1901) has found that during the divisions 



