504 C. M. Child 



degree to give rise to the characteristic structures. But the rapid- 

 ity of morphological restitution decreases as the distance of the 

 level of restitution from the original oral end increases. As I 

 have attempted to show above, this means simply that physio- 

 logical likeness to the oral end decreases with increasing distance 

 from that end. 



This change in physiological likeness with difference in level 

 is not uniform: in the oral half or two-thirds of the body it is not 

 very great, but further aborally the rapidity of oral restitution 

 decreases rapidly until it becomes zero, i.e., in the extreme aboral 

 region the distal end of the piece is incapable of physiological sub- 

 stitution for the original oral end in sufficient degree to produce 

 visible morphological results. 



That such substitution is not a purposive or adaptive act but 

 merely a necessary consequence of the constitution of the system 

 and particularly of the correlations of its parts, I have pointed out 

 elsewhere (Child 'o8a). 



In my discussion of polarity in Tubularia I called attention to 

 the fact that polarity might appear in quantitative and qualitative 

 regional differences as well as in the oral-aboral or anterior-pos- 

 terior differences which are commonly termed polar differences 

 (Child '07. These regional differences in the rapidity of restitu- 

 tion in Harenactis are as a matter of fact one expression of the 

 physiological differences along the axis which we commonly 

 group together under head of polarity. But discussion along 

 this line is postponed until after the presentation of further data. 



SUMMARY 



I. The wound-reactions and the general course of restitution 

 in Harenactis do not differ widely from those processes in Cerian- 

 thus. The contraction following the wound is certainly not purely 

 muscular: it is apparently characteristic of the tissues in general 

 and is probably, at least in part, the consequence of certain phys- 

 ical properties of the tissues. The result of this contraction de- 

 pends upon incidental factors, at least in large measure: under 

 certain conditions the contraction approximates the margins of 



