Factors of Form Regulation in Harenactis Attenuata 475 



the following conditions: First, approximation of at least certain 

 parts of the cut surfaces, in such manner that when the cells of 

 these regions become "embryonic" after the wound has been 

 made union may occur; and second, the existence in the thin mem- 

 brane thus formed of a certain degree of mechanical tension or 

 stretching. The diagrammatic Figs. 4 and 5 will serve to illus- 

 trate the point : Fig. 4 represents a region of the body-wall adjoin- 

 ing a wound, the dotted area indicating the region in which the 

 loss of the original differentiation occurs, i.e., in which the cells 

 become "embryonic." In case the cut margin is straight as in 

 Fig. 4 and no other cut surface is in contact with it, no further growth 

 occurs and the wound is never closed. The cut margin simply 

 heals over, though as I showed for Cerianthus (Child '04a, p. 70) 

 it retains for a long time' and probably indefinitely, the power to 

 unite with another cut surface if contact between the two occurs. 

 As a matter of fact, however, in most cases except extensive 

 longitudinal wounds, the contraction of the regions about the 

 wound brings about more or less "puckering" of the cut margin. 

 In cases of transverse section of the body the cut end commonly 

 appears very much as if drawn together as the mouths of bags are 

 often closed by a "draw-string." Fig. 5 is a diagram of a portion 

 of such a cut surface in a state of moderate contraction. Various 

 angles are formed between the different parts of the surface and 

 there is of course more or less elevation or depression of different 

 parts perpendicular to the plane of the figure. Under these con- 

 ditions the cells immediately adjoining the cut lose their original 

 differentiation as in the preceding case, but the process does not 

 stop here. These cells form thin membranes across the most 

 acute portions of the angles between different parts of the cut sur- 

 face, i.e., these angles gradually become less deep by the extension 

 from their apices of the thin membranes of new tissue. The 

 dotted lines in Fig. 5 indicate various stages in this process. It will 

 be noted that no appreciable growth occurs where the cut margin 

 is convex. In a case where the wound was so widely open as in 

 Fig. 5 growth of the new tissue would probably not proceed much 

 beyond the stage indicated in the figure, i.e., closure of the wound 

 would never occur unless other conditions arose. Commonly, 



