CHANGE OF SHAPE IN PLANARIA. 287 



long pieces, where it is much broader. In short pieces possess- 

 ing the old head and consequently capable of rapid and frequent 

 locomotion, the change occurs much more rapidly than in other 

 pieces and the body often becomes much more slender propor- 

 tionally than that of the " normal " animal and tapers more toward 

 the posterior end. In pieces from the posterior region of the 

 animal the chief region of attachment forms the largest part of 

 the piece. Since attachment occurs by the lateral margins as 

 well as by the tip of this region, the change of shape occurs most 

 rapidly in the regions just behind the new head, for these are the 

 regions where the greatest change in the direction of the strains 

 occurs. 



Summing up the data presented, the first point of importance 

 is that no necessary connection exists between the change of 

 shape and the redifferentiation or regeneration of parts : the two 

 processes can be separated from each other almost completely 

 in time, and are often separated in space. Secondly, it is possible 

 to control the change of shape, to inhibit it, to stop it at any point, 

 or to allow it to proceed, by controlling the locomotor activity. 

 And finally, the tissues in regions undergoing change of shape 

 show very distinct indications of physical deformation in the direc- 

 tion of the strains. The conclusion which seems to me to accord 

 most closely with the facts as they now stand is that the change of 

 shape is primarily a mechanical deformation resulting, at least in 

 large part, from the strains which arise in locomotion, these strains 

 being altered in direction and amount in pieces as compared with 

 the whole animal. The final shape, /. c., the cessation of change 

 is probably determined by the physiological reaction of the dif- 

 ferentiating or redifferentiating tissues to the altered conditions, 

 and the consequent establishment of an equilibrium. 



II. FUNCTION, FORM AND REGULATION. 



Since the appearance of my earlier papers on regulation cer- 

 tain reviews and criticisms of the work have appeared. From some 

 of these, particularly from Driesch's review (Driesch, '05) I can 

 only conclude either that I have failed to state my position clearly, 

 or that the reviewers have not become sufficiently acquainted with 

 my work to understand what that position is. Driesch, for example, 



