PHYSIOLOGICAL DOMINANCE 



103 



forms from the level b 

 of Fig. 49 give rise to 

 new tails and to all parts 

 below their own level 

 (Fig. 53)^ but never 

 produce a mouth and 

 pharynx, while headless 

 forms from the level a 

 or any level apical to it 

 give rise to mouth and 

 pharynx as well as to 

 postpharyngeal regions 

 (Fig. 52), but never to 

 regions representing 

 more apical levels than 

 themselves. 



If, however, a head 

 of any sort, even a rudi- 

 mentary, anophthalmic 

 head, like that of Fig. 

 51, with no eyes and 

 small, very incompletely 

 developed, cephalic 

 gangha, arises on a piece 

 from the level b, then 

 the regions of the piece 

 adjoining the new head 

 give rise to the parts 

 representing all levels 

 between the head and 

 the level which the piece 

 b occupied in the original 



Figs. 49-53, — Planaria doroio- 

 cephala: Fig. 49, outline indicating 

 regions a and h from which pieces are 

 taken; Figs. 50-53, different results 

 of reconstitution, depending on pres- 

 ence or absence of a new head-region. 



individual. In other words, 



