80 C. M. Child 
stitution do, however, exist in this region: the decreasing rapidity 
of disc and tentacle-formation and the increasing ability of the 
aboral end to function as a foot with approach to the original 
proximal end are indications of such quantitative differences. 
The data of oral restitution in the preceding paper show only 
quantitative regional differences, except at the extreme proximal 
end of the body: the data of aboral restitution considered above 
show only quantitative differences, except in the distal, 1. e., the 
cesophageal region. ‘These results are, however, seemingly con- 
tradictory, for a given region of the body cannot be qualitatively 
like and different from other parts at the same time. Evide-tly 
some other factor besides the constitution of the body at different 
levels is involved in the phenomena of polarity. If we regard this 
factor as directive, as determining that oral structures shall arise 
in one direction, aboral in the other, we fall into new difficulties, 
for the phenomena of heteromorphosis then require a special 
hypothesis. I believe we must consider the physiological cor- 
relations between parts as an essential factor in determining the 
phenomena of polarity. In the following general consideration of 
polarity a brief analysis of the factors and the role played by each 
is attempted. 
II POLARITY IN GENERAL 
I The Facts of Regulation and Their Significance 
The most conspicuous fact among the data of regulation, so far 
as they concern polarity, and one might add, among the phenomena 
of development, is the appearance of characteristically different 
structure-complexes at the two ends of the principal axis. In 
regulation this apparent polarization can be well illustrated by the 
formation of structures typical of one end or thé other from cells 
at exactly the same level of the body, according as these cells 
form one end or the other of the piece. Such polar differences 
occur throughout the middle region of Harenactis, Cerianthus and 
in many other forms. ‘They are of course merely an expression 
in the isolated piece of what we find in the whole in normal devel- 
opment. These facts alone seem to point to the existence of a 
