g6 INDIVIDUALITY IN ORGANISMS 



them on a layer of vaseline or other surface to which they 

 cannot attach themselves, the tissues are often so tough 

 that rupture does not readily occur, and the anterior 

 individual struggles more and more violently to free 

 itself from the hindrance which is preventing its advance. 

 In these animals such struggles often terminate in the 

 complete subordination of the posterior zooid: it is not 

 torn loose from its attachment, but lets go its hold 

 and no longer reacts independently. Later, when the 

 anterior individual has become more quiet, the same 

 procedure may occur again. Evidently as the activity 

 of the anterior individual increases the range of domi- 

 nance increases, and, if fission does not occur at once, the 

 posterior zooid may finally be brought under control. 

 Moreover, one of the simplest ways of inducing fission 

 in this species is to cut off the head of the anterior indi- 

 vidual. Such animals creep about even in the absence 

 of the head, but under these conditions the posterior 

 zooid is more completely physiologically isolated and 

 separation soon occurs if the tissues are not too tough. ^ 

 Experiments to be described below will show other 

 ways in which the existence of dominance can be demon- 

 strated and its range varied and controlled in these and 

 other animals and in many plants. 



THE INDEPENDENCE OF THE APICAL REGION 



The apical region of the organic individual is, to a 

 large extent, independent and is capable of developing, 



* For a more extended consideration of the process of fission and the 

 various indications of the presence of the posterior zooids see Child, 

 "Physiological Isolation of Parts and Fission in Planaria, "Archiv 

 filr Entwickelungsmechanik, XXX, 11. Teil, 1910; "Studies on the 

 Dynamics of Morphogenesis, etc., Ill," Jour, of Exper. ZooL, XI, 191 1; 

 "Studies, etc., VI," Archiv fur Entwickelungsmechanik y XXXV, 1913. 



