CONTROL OF HEAD-FORM IN PLANARIA 117 



lation of the piece by section exerts in some way a retarding 

 or inhibiting effect on head-formation and development. 



In the same paper (Child, '14 b) it was also pointed out that 

 section of a piece is followed by two distinct reactions, the one, 

 stimulation to a greater or less degree of the piece as a whole, 

 the other the reaction of the cells adjoining the cut surface or 

 surfaces, a process of dedifferentiation, division, growth and new 

 differentiation. Lea\'ing out of consideration for the present 

 the posterior cut surface of the piece we may distinguish the 

 region which undergoes dedifferentiation and gives rise to new 

 tissue as x (fig. 10) from the rest of the piece, y, which is temporar- 

 ily stimulated. The relation between the frequency of head- 

 formation or of the different types of anterior end developed 

 from the region x and the degree of stimulation of the region 



lO 



y may then be expressed in a general way by the formula head- 

 frequency = . This formula makes no pretensions to 



rate y 



mathematical accuracy but is merely a brief way of stating 

 the facts. 



It is necessary now to proceed somewhat further in the analy- 

 sis of -this relation between rate x and rate y. In order to give 

 rise to a new head the cells at x must first lose their differentiation 

 and approach or attain the embryonic condition and to undergo 

 this change thej^ must become to a large extent physiologically 

 independent of other parts of the piece and they must also be 

 able to grow at the expense of other parts of the piece. The 

 removal of parts anterior to the cut surface and the presence 

 of the cut surface itself are effective in all cases in inducing more 



