DOMINANCE IN RECONSTITUTION 339 



the reconstitution. Moreover, an aggregate of dissociated cells of Cory- 

 morpha and other hydroids can reconstitute a complete individual. Rate 

 of regeneration of the scyphomedusa Cassiopca is decreased by removal 

 of the marginal sense organs; but this is believed to indicate merely a 

 nervous influence on the general metabolic level, since respiration is also 

 decreased by removal of the sense organs (Gary, 19 16). 



As regards head regeneration in planarians, Flexner (1898) and Keiller 

 (19 10) have maintained that the cephalic ganglia may develop independ- 

 ently of the nerve cords in the piece. In a land planarian the new ganglia 

 are said to develop in continuity with the anterior ends of the old nerve 

 cords (Bandier, 1936). However, the distance between the cut ends of 

 the nerve cords and the locus of the new ganglia is so small and nerve 

 fibers so difficult to distinguish in planarians that a definite conclusion 

 seems impossible. Even if the ganglia develop in continuity with the 

 nerve cords, it does not necessarily follow that their development is de- 

 termined by the cords; the ganglionic development apparently deter- 

 mines reorganization of the region of the nerve cords where it develops. 

 The fact that heads can develop from a partial transverse or even a 

 longitudinal cut surface lateral to the nerve cords leaves little doubt that 

 ganglia can develop independently of the old nerve cords, though lateral 

 branches of the cords, if present at the cut surface, may play a part in 

 localizing head development. The ganglia of the regenerating head of the 

 nemertean Lineus develop in complete independence of the old nerve 

 cords and later become connected with them by outgrowth of fibers from 

 the ganglia; in some other nemerteans the gangha develop at the anterior 

 end of the cords (Coe, 1934a, h). 



It seems to be conclusively established that in the earthworm and some 

 other annelids head regeneration can occur in the complete absence of the 

 nerve cords at the cut surface.^ Nevertheless, the old nervous system may 

 play a part in localizing head regeneration. When the ventral cord is 

 present at the cut surface, it is evidently a factor in localizing head re- 

 generation in annelids.'' Regeneration of a posterior end does not take 

 place or is incomplete in absence of the nerve cord, and implanted pieces 

 of cord may localize posterior regeneration.^ In view of the data, it is 



3 Goldfarb, 1909, 1914a; Siegmund, 1928; Bailey, 1930, 1939; Avel, 1932; Kropp, 1933; 

 Okada, 1934; Crowell, 1937; Painter, 1938. 



■• Morgan, igo2h; von Haflner, 1931; Zhinkin, 1935; Sayles, 1937. See also Hyman, 1940, 

 "Aspects of regeneration in annelids," Amer. Nat., 74; Sayles, 1940, "Buds induced by im- 

 plants of posterior nerve cord, etc., "Biol. Bull., 78. 



s G. E. Holmes, 1931; Zhinkin, 1936; Sayles, 1939. 



