114 INDIVIDUALITY IN ORGANISMS 



independent of y do they begin the development of a 

 new individual by the formation of a head. The 

 formation of a head at the end of a piece is then exactly 

 the same process as the transformation of short pieces 

 into heads when no other part of the body is present 

 (pp. 96-101). The new head arises independently of 

 other parts and dominates them. The influence of 

 other parts on head-formation is merely inhibitory or 

 negative, while the influence of head-formation on other 

 parts is determinative or positive. The process of 

 development of the cephalic ganglia in the head formed 

 on a piece also indicates the independence of the head. 

 The ganglia arise in the new tissue independently of the 

 parts of the nervous system in the old tissue of the piece 

 and become connected with these parts only secondarily.^ 

 This fact suggests that head-formation actually depends 

 upon the establishment of a metabolic gradient in the 

 region x with its apical region near the free end and 

 decreasing in rate toward y. If this occurs, a head forms, 

 but if rate y is high enough in relation to rate %^ head- 

 formation is inhibited or retarded b)r the interference 

 between two gradients in opposite directions. Inhibition 

 or retardation of head-formation consists then in the 

 interference of one metabolic gradient with another in 

 the opposite direction or the obliteration of the one by 

 the other. 



The new basal end of the piece develops from a group 

 of cells 2, Fig. 58, which react to the wound at the basal 

 end by more or less dedifferentiation and growth. This 



^ S. Flexner, "The Regeneration of the Nervous System of Planaria 

 torva," etc., Jour, of MorphoL, XIV, 1898; Child and McKie, "The 

 Central Nervous System in Teratophthalmic and Teratomorphic Forms 

 of Planaria dorotocephala," Biol. Bull., XXII, 191 1. 



