PHYSIOLOGICAL DOMINANCE 119 



reaction involves more growth than in Planaria, and in 

 such cases considerable outgrowths may sometimes arise 

 which are neither heads nor tails, but cell masses of 

 indeterminate character which gradually differentiate 

 in relation to adjoining parts and may finally show both 

 apical and basal characteristics. 



In many of the flatworms and various other forms 

 only an apical cut surface above a certain level of the 

 body gives rise to a head, while tails may arise from cut 

 surfaces at any level basal to the head of the parent 

 body. In some of these cases the level where head- 

 formation ceases lies a considerable distance from the 

 cephalic ganglia, while in other cases head-formation 

 does not occur when the cephalic ganglia are removed; 

 but when parts of the head are removed leaving a por- 

 tion of the cephalic ganglia intact — sometimes half or 

 more, sometimes only a small part, is necessary — such 

 parts develop again. In the headless pieces there may be 

 more or less outgrowth at the apical end of the piece, 

 but it is indeterminate in character. Some authors 

 have maintained that in such cases the cephalic ganglia 

 or the more apical regions of the longitudinal nerve 

 cords exercise a specific formative influence of some 

 sort and so determine the development of a new head, 

 but there is no real evidence in favor of this view. 

 Probably the head fails to develop in such cases either 

 because the cells reacting to the wound do not attain 

 a high enough metabolic rate to become independent 

 of other parts and their development into a head is 

 therefore inhibited, as in the headless pieces of Planaria, 

 or because these cells do not dedifferentiate to a suffi- 

 cient extent to be capable of giving rise to a new cephalic 



