DOMINANCE IN RECONSTITUTION 351 



fed than in starving animals, until the equilibrium is approached or at- 

 tained. Partial individuals approach an equilibrium different from entire 

 reconstituted individuals. In acephalic forms, for example, a posterior 

 zooid develops, even in very short pieces, because the dominant head re- 

 gion is absent, and this zooid increases in length at the expense of the 

 region anterior to it in starving acephalic forms, often becoming relatively 

 very long. In starving intact animals different parts do not undergo re- 

 duction at the same rate. Apparently the most active, or most contin- 

 uously active parts decrease least rapidly because they are able more 

 nearly to maintain themselves at the expense of other parts than less 

 active regions. The head decreases in size less rapidly than the body; 

 but most of the digestive tract, in which the level of functional metabolism 

 is undoubtedly very low in the absence of food, may disappear com- 

 pletely. With repeated reconstitution of starving animals scale of organi- 

 zation may become very small (S. J. Holmes, 191 1). In short, with the 

 progress of experimental analysis reconstitution of large individuals from 

 long, and smaller individuals from shorter, pieces and the changes in form 

 and proportion appear somewhat less mysterious than they have appeared 

 to certain authors in the past. They are evidently expressions of physio- 

 logical factors that can be altered and controlled experimentally. Scale 

 of organization and morphological type of head depend upon degree of 

 activation or inhibition of cells concerned in its formation, and this affects 

 its rate of development and its dominance ; differential growth in relation 

 to gradient-level and available nutritive supply appears to be the essen- 

 tial factor in the change in form and proportion. 



The distance between regenerated planarian head and pharynx, that is, 

 scale of organization of the prepharyngeal region, can be altered experi- 

 mentally in pieces from the postpharyngeal region. In pieces representing 

 this region (Z of Fig. 119, ^) the position of the pharynx reconstituting 

 in normal environment is approximately that indicated in Figure 119, B, 

 that is, somewhat anterior to the middle. Under inhibiting conditions — 

 low concentrations of anesthetics and other toxic agents, or culture water 

 fouled by dead planarians — head regeneration is inhibited, the head is 

 small, and scale of organization of prepharyngeal and pharyngeal regions 

 decreases according to degree of inhibition of the head (Fig. 119, C, D). 

 In extreme cases no pharynx develops, and there is no reorganization of 

 a prepharyngeal region (Child, 191 if, 19296). Reconstitution at higher 

 temperatures (26°-28° C.) is more rapid; heads are larger; and scale of or- 

 ganization of prepharyngeal and pharyngeal regions is increased. In forms 



