312 C. M. CHILD 



a certain size the death rate increases toward the posterior end 

 of the first zooid, i.e., independent existence is possible in smaller 

 pieces from the anterior than from the posterior regions of the 

 first zooid. 



All of these facts indicate that a graded difference of some sort 

 in the dynamic processes exists along the axis, and together with 

 the results concerning size of pieces, the experiments show, first, 

 that a certain minimal portion of this graded difference is neces- 

 sary for the regulation of a piece into a whole, and second, that 

 the rate of gradation along the axis differs in different regions of 

 the body. In other words, the minimal length of pieces which 

 are capable of forming a whole is different in different regions 

 of the body. In a later paper, however, I shall show that these 

 factors are by no means constant, but depend, at least in large 

 degree, upon the rate of metabolism or of certain metabolic pro- 

 cesses. 



It is this dynamic gradation along the axis, together with the 

 complex of correlative conditions associated with it, which I 

 regard as constituting physiological polarity. According to this 

 idea polarity is not a condition of molecular orientation, but is 

 essentially a dynamic gradient in one direction or different grad- 

 ients in opposite directions along an axis, together with the condi- 

 tions, and particularly the physiologic correlation between 

 different parts along the axis, which must result from the existence 

 of such a gradient or gradients. 



The change from the production of wholes to partial structures 

 with decreasing length of piece constitutes strong evidence in 

 favor of this view, while on the basis of a molecular hypothesis 

 of polarity it is difficult to understand whj^ short pieces should 

 not produce wholes as well as long pieces, if they produce any- 

 thing. 



The results of these experiments also indicate clearly that the 

 dynamic gradient is not uniform but differs at least quanti- 

 tatively in different regions of the body. A molecular hypothesis 

 of polarity affords us no basis for understanding why the minimal 

 piece for the formation of a whole must be larger in some regions 

 than in others. 



