Factors of Form Regulation in Harenactis Attenuata 477 



approaches a straight Hne or becomes convex, is of special inter- 

 est, in that it shows very clearly the dependence of the process 

 upon physical conditions of some sort. Moreover, the thin mem- 

 brane when formed is always under a certain degree of tension, 

 and this condition together with the other facts again suggests 

 surface tension. While this factor is apparently sufficiently 

 important to determine under certain conditions whether growth 

 of new tissue and wound closure shall occur or not, the process of 

 growth itself, when it does occur, is of course very different from 

 the behavior of a fluid or semi-fluid film. After the wound is 

 closed the mechanical conditions of tension favorable to further 

 growth arise from the distension of the enteron with water. 



Thus far it has been impossible to discover any very great 

 regional physiological difference either in the method or the rate 

 of wound closure, though apparently the rapidity of the reactions 

 is greatest in the oral regions and decreases aborally. In a given 

 region of the body the method of closure of aboral wounds does 

 not differ essentially from that of oral wounds. Certain inciden- 

 tal regional differences in the method and rate of wound closure 

 do, however, appear; these are due primarily to the anatomical 

 structure of the animal, and secondarily to the general occurrence 

 of contraction of the tissues as a wound reaction. The factors 

 chiefly concerned in these differences are briefly considered in the 

 following sections. 



I The Mesenteries 



The contraction following the wound involves not only the body- 

 wall but any mesenteries which may have been injured. In a 

 terminal wound in the oesophageal region, for example, the mesen- 

 teries extend from body-wall to oesophagus, and after section the 

 mesenteries contract, as well as the body-wall and the oesophagus. 

 It is this contraction of the transverse cut surface of the mesen- 

 teries that brings the cut margins of the body-wall and the oesopha- 

 gus together with such uniformity in oesophageal regions (Figs. 7 

 and 8). In consequence of the presence and arrangement of the 

 mesenteries, it is impossible for the cut end of the body-wall to 

 close over the end of the oesophagus. In all cases where section 



