6 J. WILLIAM BUCHANAN 



rary stimulation of the piece in all regions not directly injured by 

 the cuts, the degree and duration of stimulation depending on 

 the length of the piece and its original position in the gradient. 

 For convenience in discussion the cells near the anterior cut 

 surface that are directly involved in the wound stimulation and 

 concerned in head formation may be designated as the X cells 

 (fig. 2) and their rate of metabolism expressed as Rate X. The 

 cells of the remainder of the piece that are affected by the ner- 

 vous stimulation msiY be designated as the Y cells and their rate 

 of metabolism expressed as Rate Y. The relation to each other, 

 during this period of stimulation, of these two new conditions 



Fig. 2 An isolated piece from the body of Planaria dorotocephala. A', the 

 region adjacent to the anterior cut surface directly affected by the injury of 

 cutting; Y, the remainder of the piece, exclusive of the region adjacent to the 

 posterior cut surface. 



set up by cutting and isolation has been shown by Child to be 

 the factor determining whether or not a head shall develop. In 

 the first place, it has been shown that head determination, i.e., 

 the establishment of the conditions which determine whether or 

 not a head shall develop, occurs very soon after isolation and 

 during the period of stimulation of the Y region by section. Sec- 

 ond, head frequency decreases and the degree of stimulation of 

 the Y region increases with decrease in length of the piece with 

 anterior end at a given level. Third, it has been shown that 

 in pieces of equal length from different levels the degree of stimu- 

 lation of the Y region increases and the head frequency decreases 

 as the level from which the piece is taken becomes more posterior. 



