Form Regulation in Harenactis 87 
nomena of polarity in organisms, for both constitution and correla- 
tion, or in other words, both structure and function are factors 
in determining the character of the phenomena. 
I use the term “correlation” in its physiological sense as includ- 
ing all effects of conditions and processes in one part upon another 
part. It is only within recent years that correlations have become 
in any real sense objects of investigation. Concerning their 
nature we learn but slowly: we know that some of them are 
chemical, others physical, and still others complex, but there 
seems to be no general recognition among zodlogists such as exists 
among botanists of their great importance in morphogenesis. 
I might cite in this connection various misconceptions and mis- 
interpretations of my own work on regulation, for they furnish 
abundant proof of this fact, but time and changed conceptions 
of the problems of morphogenesis will perhaps accomplish what 
discussion cannot. 
2 Constitution and Correlation in the Phenomena of Polarity 
We may say in general that so far as internal factors are con- 
cerned, the constitution of a part at any given time determines 
what reactions are possible in it at that time, and its correlations 
determine what reactions actually occur. In Harenactis, in Plan- 
aria and many other forms the cells of a given level, e. g., the mid- 
dle of the body, possess a constitution which is capable of giving 
rise to the structures characteristic of either pole. But whether 
they actually form tentacles or a foot, a head or a tail in any partic- 
ular case is very evidently determined by their physiological rela- 
tions with other parts, i. e., their correlations. Manifestly the 
cells of a given level must be in a different physiological state 
when they are in correlation only with parts aboral or posterior 
to them, from that which exists when they are in correlation only 
with parts oral or anterior to them. ‘The nature of the differences 
between these two states we do not at present know, but we cannot 
doubt their existence. Moreover, both of them are different from 
the original state when the cells were correlated with both regions 
of the body. 
