REGULATION AND FISSION IN PLANARIA. 119 



I believe that these differences are also indicative of the 

 physiological conditions at different levels. In previous papers l 

 I have suggested that the relation between redifferentiation and 

 regeneration proper depends upon the physiological differentiation 

 of parts. As the difference in physiological or functional speci- 

 fication between the part removed and the part remaining increases 

 the amount of redifferentiation decreases and that of regeneration 

 increases up to a certain limit and vice versa. To take a con- 

 crete case : the more " head-like" in the physiological sense the 

 anterior portion of a piece of Planaria is, the smaller the amount 

 of new tissue formed at this end and the greater the amount of 

 redifferentiation. Now the anterior ends of pieces from the 

 anterior regions of the body of Planaria and those from the pos- 

 terior half of the postpharyngeal region are visibly more " head- 

 like" than those of pieces from the middle region. This is 

 evident from the degree and character of their activity. Corre- 

 spondingly we find that only the part anterior to the eyes is 

 formed by regeneration, the "neck" and in some cases the 

 auricles themselves being formed by redifferentiation in the 

 pieces from anterior and posterior regions (Figs. 9, a, c, and 10, 

 a, c}. On the other hand, in the pieces from the middle region 

 not only the head but a longer or shorter " neck" is formed by 

 regeneration. 



The difference in size of the heads furnishes more evidence 

 along the same line. The greater the relative intensity of a 

 given functional complex the larger the relative size of the struc- 

 tural complex which represents it. Hence the more " head-like " 

 a given piece is the larger the relative size of the head formed 

 and vice versa (cf. Figs. 9, a, b, c, and 10, a, b, c]. 



The conclusions from this line of evidence are the same as 

 those reached from a comparative study of pharyngeal position 

 viz., that in the posterior half or two thirds of the postpharyn- 

 geal region a marked increase in the functional conditions charac- 



1 "Studies on Regulation VIII., Functional Regulation and Regeneration in 

 Cestoplana," Archiv f. Entivickelnngsmech., Bd. XIX., H. 3, 1905. 



" Contributions toward a Theory of Regulation I., The Significance of the Dif- 

 ferent Methods of Regulation in Turbellaria," Archiv f. Enlwickeluiigsniech., Bd. 

 XX., H. 3, 1906. 



