HEAD-FREQUENCY IN PLAN ARIA 413 



b under the head 'teratophthalmic ' as in the other columns. In 

 series 498 with well-fed animals the level of head-frequency is 

 higher in all pieces than in series 302 with animals starved eight 

 days and only moderately fed for several weeks previously, but 

 the differences between I and II are distinct in every case. 

 Moreover, the short period of stimulation in series 498 is appar- 

 ently about as effective in increasing head-frequency as the long 

 period in series 302. According to the evidence from head deter- 

 mination (Child, '14 d), this is to be expected, but it is of interest 

 to find the data confirming expectation. 



DISCUSSION 



Extensive investigation of the conditions determining and af- 

 fecting head formation in isolated pieces of Planaria has led to 



Figure 3 



certain conclusions concerning the physiological relations of the 

 new head to other parts of the piece. 



It is a well-known fact that regulatory development at the 

 anterior end of a piece begins with the formation of a head, what- 

 ever the level from which the piece is taken, and that the parts 

 which normally lie between the head and the level represented by 

 the piece are formed later by reorganization of regions posterior 

 to the new head and never develop unless at least a rudimentary 

 head is formed first (Child, '11 c). In short, the head seems to 

 arise as something more or less independent physiologically of 

 other parts of the piece, while the further reogranization of the 

 anterior regions of the piece occurs only under the influence of 

 the new developing head. 



Moreover, it has been shown that the relation between the 

 cells from which the new head develops (fig. 3, x) and the rest of 



