422 S. J. Holmes 



the regulation of organic form. "A disturbance of the physio- 

 logical processes," he says, "results in varied growth activities. 

 Some of these will relieve the disturbance; the variations then 

 cease and the processes are continued." The result of the selec- 

 tion of those growth processes which relieve the disturbance— or 

 we might say make for functional equilibration — is the restoration 

 of the lost part. Jennings has not attempted to develop a theory 

 of form regulation in detail farther than to show the fundamental 

 similarity of the method of regulation in various fields, but he 

 holds that mv own point of view so far as form regulation is con- 

 cerned is in "essential agreement" with his. 



Child' has recently outlined a theory of regulation which, as he 

 states, is "somewhat similar to that adopted by" myself, although 

 differing in certain important particulars. According to both 

 Child and myself, regeneration and other formative processes 

 are the result of functional activity, or more specifically, func- 

 tional equilibration. Tissue differentiates in the direction of the 

 missing part because it takes on the functional activity of the miss- 

 ing part. To cite an illustration by Child, "after removal of 

 the anterior or posterior end in Bipalium or Planaria maculata 

 the terminal regions of the piece remaining are subjected to condi- 

 tions somewhat similar to those existing in the terminal regions 

 of the part removed. The anterior end of the headless piece of 

 Planaria is subjected to external conditions more or less similar 

 to those to which the old head was subjected; moreover, its rela- 

 tion to the other parts of the body is more or less like that of the 

 head. Stimuli resulting from forward movement affect it first, 

 and are transmitted from it to other parts, etc. Functionally 

 speaking, it serves in some degree as a head. The case is similar 

 as regards the posterior end. After removal of the original pos- 

 terior end, the posterior region of the piece functions in some 

 degree as a posterior end, or to put the matter more strictly, its 

 functional relations with other parts are more or less similar to 

 those of a 'tail' or posterior end. * * * 



" Redifferentiation occurs as a result of a functional substi- 



^.■\rchiv fiir Entwickelungsmechanick, xx Band, iqo6; and this Journal, vol. iii, l')o6. 



