Factors of Form Regulation in Harenactis Attenuata 503 



to at least many forms with well marked polarity. In such forms 

 the anterior or oral end and the regions adjoining it are commonly 

 the regions of most rapid or most intense reaction to external con- 

 ditions, and are usually more frequently or more continuously 

 affected by these conditions and their changes. In short the 

 anterior or oral end is commonly dominant functionally in the 

 body (Child '08a). Regions adjoining this pole share to a greater 

 or less extent in its activity, and their physiological character is 

 more or less completely determined by their correlations with it. 

 In the more complex forms these correlations may be very definite 

 in character and localization, and structural localization may be 

 correspondingly definite, but in simple forms like the actinians the 

 degree of correlation between the anterior or oral region and other 

 parts is appatjpntly more or less nearly proportional to the dis- 

 tance between them, i.e., the greater the distance between the 

 oral end and a given region the less the physiological similarity 

 between them. If the body of such a form, e.g., Harenactis, be 

 cut into pieces the oral end of each piece is, so far as "oral proc- 

 esses" are concerned, the dominant region. Since visible morpho- 

 logical differentiation is to be regarded as the expression of the 

 functional processes in the system, the development of the morpho- 

 logical structures characteristic of an oral end may be expected to 

 occur in the most oral region of the piece, if anywhere. Such 

 development will occur, provided the region is sufficiently similar 

 to the original oral region in its physiological capacities or becomes 

 sufficiently similar in consequence of its new position and correl- 

 ations as the most oral region of the body. No exact limit can be 

 estabHshed for the occurrence or non-occurrence of restitution in 

 a given case: we can only say that if the region in question is or 

 becomes so far similar physiologically to the part removed, that 

 it can take the place of the latter to a certain extent in the system, 

 it will develop a morphological structure approaching that of the 

 part removed. The completeness of the restitution will depend 

 upon the completeness with which the substituted region takes 

 the place of the old in the system. 



In Harenactis all levels of the body except the extreme aboral 

 region are capable of substitution for the original end in sufficient 



