PHYSIOLOGICAL DOMINANCE 



93 



The separated posterior portion becomes a new animal 

 while the anterior portion develops 

 a new posterior end, and fission is 

 sooner or later repeated. There is 

 no morphological indication of a 

 second individual or zooid in the 

 posterior region of the body, but 

 one or more such individuals are 

 indicated by the metabolic gradient 

 of the major axis and by various 

 other physiological differences. 

 The apical region of this gradient 

 is the head of the animal, and from 

 the head the metabolic rate de- 

 creases to the level where separa- 

 tion occurs in fission; there a 

 sudden rise in rate occurs, and then 

 again a downward gradient toward 

 the posterior end. The region 

 where the rate rises suddenly 

 represents the apical end of the 

 second individual and the down- 

 ward gradient following is the 

 gradient of the major axis of this 

 zooid. In the shorter animals 

 only one of these zooids is present, 

 but as the length increases the 

 basal body region may show two, 

 three, or more of these distinct 

 gradients. Represented graphi- 

 cally the metabolic gradient in such 

 an animal is like the curve in Fig. 45 ; a is the head-region, 



Fig, 44. — Planaria 

 dorotocephala , outline, 

 indicating several zooids 

 in basal region; ff, usual 

 level of fission. 



