124 INDIVIDUALITY IN ORGANISMS 



part in determining the characteristic shape of the 

 animal they undoubtedly play a part in determining 

 the approach to this shape in pieces undergoing recon- 

 stitution, but in cases where they are not primarily con- 

 cerned the metabolic relations are unquestionably the 

 primary factors in determining shape and proportions 

 of the whole and parts. 



In most adult animals and embryonic stages which 

 are capable of any considerable degree of reconstitutional 

 reproduction, a limit of size of isolated pieces seems to 

 exist below which reconstitution becomes incomplete or 

 fails to occur. In Planaria, for example, with decrease 

 in size of piece head-frequency falls to zero, but with still 

 further decrease in size head-formation begins to occur 

 again and head-frequency rises. These changes are 

 simply due to changes in the relation StU (see pp. 109- 

 10). With decreasing size of the piece, y is more and 

 more highly stimulated by section until in pieces below 

 a certain size heads do not develop at all, but when 

 the piece becomes very small y practically disappears, 

 for the whole piece becomes involved in the direct 

 wound reaction and so corresponds to the region x 

 or such a region in relation to both cut ends. In such 

 pieces there is nothing to inhibit or retard head- 

 formation except the simultaneous development of a 

 head at the opposite end (see pp. 98-101), and in 

 such cases the effect is mutual and results merely in 

 retardation. 



Here then the completeness or incompleteness of 

 reconstitution in relation to size of piece is wholly a 

 matter of quantitative metabolic relations. There is 

 no minimal size of piece which represents the ''organi- 



