DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I 177 



is greater anteriorly than posteriorly; after 14 days in cyanide the an- 

 terior-posterior ratio in the same pieces is considerably higher than in the 

 control, the conditioning of the anterior region to cyanide being appar- 

 ently greater than that of the posterior. These differences are directly 

 visible and often become greater in later stages, but these have not been 

 measured. With some agents — for example, ethyl alcohol — the secondary 

 modification occurs so rapidly that the differential inhibition preceding it 

 is slight or scarcely appreciable. In general, the greater susceptibility of 

 the regenerating head region than of the posterior regenerate to lethal 

 concentrations or dosages and its more rapid or greater conditioning in 

 a lower range of concentrations are conspicuous characteristics of planar- 

 ian pieces undergoing reconstitution. That they are expressions of a longi- 

 tudinal pattern appears beyond question. 



HEAD FORMS AND HEAD FREQUENCIES IN PLANARIAN RECONSTITUTION 



In the reconstitution of pieces of a number of planarian species graded 

 series of head forms develop, ranging from the normal fully developed 

 head to the completely acephalic condition. These head forms are evi- 

 dently expressions under different conditions of a differential or gradient 

 pattern in the parent body and persisting in the regenerating head with 

 both anteroposterior and mediolateral components. On the one hand, 

 the head forms constitute a continuous series of differential inhibitions of 

 head development; on the other, a series of secondary modifications rep- 

 resenting differential conditioning and recovery. Chief attention is given 

 to the inhibition series because of its relation to physiological, as well as 

 external, factors, because the degree of differential inhibition and conse- 

 quently the head form can be experimentally altered and controlled in 

 many ways, and because this series has thus far provided a basis for a 

 wide range of experimental analysis. The secondary modifications, though 

 physiologically equally significant, appear under rather narrowly limited 

 experimental conditions; they will be considered in a later section. 



Head forms. — The head forms of the inhibition series have been classed 

 in five groups for convenience, but it must be remembered that the limits 

 of these groups are arbitrary and that the series is actually continuous. 

 The groups are as follows: 



The normal head: The head form which is typical of the species in 

 natural environment is triangular in general outline, with two separate, 

 bilaterally localized "eyes" (photoreceptors) and two cephalic lobes (chem- 



