Form Regulation in Harenactis 93 
¢ Quantitative axial or polar differences: in heteromorphosis 
the heteromorphic structure is very often delayed in development. 
d Quantitative regional differences: when axial phenomena 
are not complicated by other factors, the rapidity of restitution and 
often the size or completeness of the parts decreases as the distance 
between the level at which they form and the similar pole of the 
original individual increases. 
These phenomena may be complicated and altered by various 
factors such as greater differentiation or specification of certain 
parts and consequent decreased power of restitution, the intensity 
of the restitutional stimuli under different experimental condi- 
tions, cephalization of the nervous system, etc. 
7 The regional differences, both qualitative and quantitative, 
are determined by the regional differences in constitution existing 
in the original individual at the time the pieces were isolated. 
They are the result of the structural polarity which is a regional 
difference in constitution along the axis. 
8 The axial or polar differences, both qualitative and quanti- 
tative, are determined by the correlations existing between the 
parts of the piece after its isolation. ‘They are the result of the 
physiological polarity. 
NOTE 
Since this paper was written the results of Lillie’s recent 
centrifuge experiments on Chztopterus eggs have appeared.! 
Lillie’s conclusions require a brief consideration in connection with 
what has been said above concerning polarity. 
The persistence of the original polarity and symmetry in the 
“ground substance” after centrifuging, and irrespective of the 
position of the granules, has led Lillie to suggest that organic polar- 
ity and symmetry are molecular and essentially similar to the polar- 
ity and symmetry of crystals. According to this hypothesis 
polarity is directive, 1. e., it is the result of molecular orientation. 
T have endeavored to show above that such a hypothesis cannot 
account for the regional phenomena along the axis, which are 
observed in regulation both in young and adult forms. Lillie 
assumes that these differences are due to secondary factors. As 
1 Biol. Bull. xvi, 2, 1909. 
