Factors of Form Regulation in Harenactis Attenuata 479 



are drawn together and unite (Fig. 11). In this manner the 

 lateral mouths and lateral partial discs are formed. 



If, however, the lateral wound is not sufficiently deep to involve 

 the oesophagus, or if it is in the suboesophageal region, closure 

 occurs, if it occurs at all, by approximation of the cut margins of 

 the body-wall (Fig. 12). If the wound is not deep enough to sever 

 the mesenteries of the side of the body, their contraction aids in 

 bringing the cut edges together and closure occurs in a very short 

 time. This is always the case in transverse wounds in the oesopha- 

 geal region which do not involve the oesophagus, and in the sub- 

 oesophageal region when the mesenteries are not completely 

 severed. If, however, a wound in the suboesophageal region is 

 deep enough to sever some of the mesenteries their contraction 

 can no longer aid in bringing the cut margins of the body-wall 

 together. As a matter of fact, such wounds are commonly rather 

 slow in healing and closure usually occurs from each end of the 

 transverse wound toward the middle. Since the body is cylin- 

 drical it is evident that a lateral transverse wound varies in depth 

 from each end toward the middle, being deepest in the middle. 

 In those regions of the wound where the mesenteries are not com- 

 pletely severed their contraction will aid in bringing the cut edges 

 of the body-wall together, while in other regions no such factor 

 will exist. Consequently closure at the ends of such a wound will 

 occur much more readily and rapidly than elsewhere. Frequently 

 the middle regions of these lateral wounds remain open for a long 

 time, since the cut surfaces are not sufficiently approximated for 

 the formation of new tissue between them. 



Longitudinal wounds of any considerable length in the body of 

 Harenactis very often remain open indefinitely. The margin of 

 the body-wall on each side usually rolls inward spirally so that 

 approximation of the cut edges is impossible, or in some cases one 

 margin precedes the other and the whole body rolls into a single 

 spiral with longitudinal axis. In either case closure of the wound 

 can never occur. The usual failure of the cut margins of the body- 

 wall to approximate each other after a longitudinal wound is due 

 simply to the fact that there is nothing in the structure of the body 

 which serves to draw the margins together or to prevent their 



