CHANGE OF SHAPE IN PLANARIA. 293 



terior piece becomes visibly "posterior" as regards the character 

 of its movements before any marked structural changes occur, 

 regulation must have occurred before such changes in movement 

 could appear. 1 



3. Mechanical Regulation and Morphallaxis? 



In the first of the " Studies " the idea of mechanical regulation 

 as the chief factor in the change of shape in pieces of Stenostoina was 

 developed. According to my conception mechanical regulation is 

 primarily a mechanical deformation of physically plastic tissues. 

 Whether we consider regulation as a return or approach to the 

 normal condition after a disturbance of this condition (Driesch, 

 '01), or as a return or approach to a condition of physiological 

 equilibrium after a preceding condition of equilibrium has been 

 disturbed (my own definition), mechanical regulation in my sense 



1 In my paper I referred to these changesas functional regulation and called atten- 

 tion to the altered character of the movement as indicating that they had occurred, 

 but while the movement is undoubtedly a factor in what follows, it would be absurd 

 to suppose that it is the primary factor in such a case. 



- Some years ago, in describing the course of regulation in Planaria macttlata, 

 Morgan called attention to the peculiar changes in shape which the pieces undergo, 

 these changes consisting mainly in a decrease in width and an increase in length. 

 Concerning this process he says : " Thus the relative proportions of the planarian are 

 attained by a remodelling of the old tissue. I would suggest that this process of 

 transformation be called a process of morphallaxis " (Morgan, '98, p. 385). Later 

 ('oo) he applied the same term to the changes in shape and proportion in Hydra and 

 other forms. So far as I am aware, Morgan has not stated at any time whether the 

 term "morphallaxis" is to be applied to the whole process of form regulation in 

 Planaria and other forms, including the regeneration and redifterentiation which 

 occurs, or only to the changes in shape and proportions of the piece. From the quo- 

 tation given above it would appear that he intended it to apply only to the change in 

 shape and proportions, but in his latest statement ('07, p. 15) he apparently applies 

 the term to the whole process of form regulation by redifterentiation. In my earlier 

 "Studies " I used the term with reference only to the changes in proportion and 

 shape : later, however, I substituted "change in proportions " for it as less ambigu- 

 ous for my purposes (Child, '050, p. 253). Driesch ('oi) considered the term as 

 synonymous with " Restitution durch Umdifferenzierung," a very different meaning 

 from that which I had given it in my work. It is probable that Driesch's misunder- 

 standing of my conclusions concerning form regulation is in part due to our different 

 interpretations of this term. 



In several of my earlier papers the word "form" was used for " shape" and 

 " outline." I agree with Driesch that this use of the word may be misleading, but I 

 was careful to distinguish between form in this sense and structure, and pointed out 

 that the factors concerned with change in form probably had nothing to do in many 

 cases with change in structure (Child, '02, p. 218). 



