310 C. M. CHILD 



cells of a given level become less and less capable of the regula- 

 tory reaction which leads to formation of a whole. I believe that 

 this is strictly true, moreover, this conclusion ^s in complete 

 agreement with what occurs as regards the character of regula- 

 tion with decreasing size of the piece. We see pieces becoming 

 less and less capable of forming wholes and it is evident that this 

 decreasing capacitv is not due to lack of material, for in many 

 cases the partial structure which such pieces form apparently 

 requires as much or more material than the formation of 

 whole and, what is still more striking, we often find that a small 

 piece which produces a partial structure in the first regulation, 

 will, after another operation at the incomplete end, form a normal 

 whole. In a headless tail, for example, the anterior end is physio- 

 logically more capable of head formation after the first regula- 

 tion of the piece is completed, even though it does not form a 

 head, and if we remove the anterior end a head will be formed in 

 the same manner as in pieces from the anterior region of the body. 

 These pieces will be considered more fully in another paper. 



Furthermore, the decrease in the rapidity of regulation with 

 decrease in size below a certain limit is not due to increasing- 

 difficulty in obtaming the necessary material, for such pieces 

 often accomplish much more extensive growth than larger pieces. 

 In a later paper I shall show that a decrease in size of the piece 

 is accompanied by a decrease in rate or intensity of the dynamic 

 processes and that the decrease in regulatory capacity is directly 

 connected with this change. 



Lastly the increasing frequency of death with decreasing size 

 of the piece points to the conclusion that not only the ability of 

 the pieces to form wholes, but their ability to remain alive is 

 dependent to a considerable degree upon the correlation between 

 parts of more or less different character. 



All the facts then point toward one general conclusion which 

 may be stated briefly here, viz: that the capacity of the cells or 

 parts at any level of the body is dependent, not jnerely upon their own 

 constitution, and upon conditions which permit continued existence 

 hut upon physiologic correlation with other parts more or less dif- 

 ferent from them. As the length of a piece of Planaria decreases. 



