CONTROL OF HEAD FORMATION IN PLANARIA 7 



Fourth, in young animals the rate of metaboHsm is higher than 

 in old animals and the difference between Rate X and Rate Y 

 is therefore not as great as in old animals and the head frequency 

 in corresponding fractions of bod}' length is less in young than 

 in old animals. Fifth, the stimulation of the Y region can be 

 eliminated by exposure for a few hours after section to dilute 

 KNC and in pieces in which this stimulation is sufficient to in- 

 hibit head development, such treatment increases head frequency 

 (Child, '14 a, '14 b, '16, '20 a). The facts indicate very clearly 

 that in some way physiological conditions in Y tend to inhibit 

 head formation in X and that this inhibiting action increases 

 as the metabolic activity of Y increases in relation to that of X 

 and vice versa. 



A brief consideration of the conditions resulting from the 

 isolation of a piece will serve to throw some light on this relation 

 between X and Y. The result of section is not simply the locahza- 

 tion of a cut surface at X with its effect on adjoining cells, but 

 also the isolation of the cells at X from the influence of all phys- 

 iological correlative factors originating in regions anterior to 

 the level of the piece. Since these factors played a large part 

 in determining the differentiation at X, their absence must tend 

 to bring about dedifferentiation in these cells. But the region 

 X is physiologically continuous with the region Y which is less 

 directly affected by isolation of the piece and retains more or 

 less completely its differentiation. Consequently, any cor- 

 relative factors originating in Y and affecting X must tend to 

 keep the X cells differentiated to some degree, i.e., to retard or 

 inhibit their dedifferentiation. The greater the physiological 

 activity of Y, the more effective are these factors and therefore 

 the greater the degree of inhibition of head development and 

 vice versa. 



Since the experimental data already at hand indicate that 

 the factors in this relation between X and Y which are concerned 

 in head development are essentially quantitative and closely 

 associated with the rate of metabolism, particularly of oxida- 

 tions, in the two regions, we may say that head frequency varies 

 in general with the ratio of Rate X to Rate Y or with the value 



