126 THE ORIGIN OF THE NERVOUS SYSTEM 



developmental stages, is independent of other parts. 

 In all except the simplest animals the cephalic central 

 nervous system is the primary organ of the head region. 

 If the head region is self-determining with respect to 

 other parts, certainly the same must be true in large 

 measure for the cephalic nervous system. 



In this connection it is of interest to note that Flexner 

 (1898) and Keiller (1910) have shown that in the 

 regenerating head of Planaria the ganglia may develop 

 quite independently of the cut ends of the nerve cords 

 in the piece and the connections are established later 

 by outgrowth from the new ganglia rather than from 

 the nerve cords. The development of the connections 

 between the new ganglia and the old cords in certain 

 abnormal types of head in Planaria also points to the 

 same conclusion (Child and McKie, 191 1). Moreover, 

 Goldfarb (1909) removed the nerve cord from several 

 segments at the anterior cut end of pieces of earthworm 

 and found that the head and the ganglia developed nor- 

 mally in such pieces, and the nerve cords grew poste- 

 riorly from the new head into the old parts, although 

 some regeneration in the anterior direction from the 

 end of the old cord occurred. In this case it is demon- 

 strated beyond question that the ganglia in the regener- 

 ating head develop independently of other parts of the 

 nervous system in the piece. 



There seems then to be no escape from the conclusion 

 that, at least in the lower animals and as regards its 

 earlier stages, the apical or cephalic portion of the cen- 

 tral system develops independently of subapical or 

 postcephalic regions. Its physiological dominance and 

 its independence are simply two aspects of the same 



