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 Pfliiger 
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, 19096). 
Giardina (1901) has found that during the divisions 
