102 INDIVIDUALITY IN ORGANISMS 



DOMINANCE AND SUBORDINATION IN EXPERIMENTAL 

 REPRODUCTION 



The existence of a relation of dominance and sub- 

 ordination along the major axis is shown by the fact 

 that, while the apical region is independent, other levels 

 of the body can develop only in organic connection with 

 more apical levels or with the apical region itself. In 

 Tubularia and related forms stolons arise only in relation 

 to stems or hydranths and stems, stem regions appear 

 only in relation to higher levels in the gradient, etc. 

 Stolons may grow out from stems in the absence of 

 hydranths, and under certain conditions when the meta- 

 bolic gradient is slight stolons may even arise at both 

 ends of a piece of stem, but no case has ever been 

 observed of the development of a stolon independently of 

 other more apical levels. 



This relation is also very evident in Planaria.* 

 The reconstitutional development of pieces from the 

 middle and posterior regions of the anterior individual, 

 such as a and b in Fig. 49, ranges according to the 

 physiological condition of the animal and with experi- 

 mental conditions from a normal complete animal like 

 Fig. 50 through various intermediate forms, of which the 

 anophthalmic is shown in Fig. 51, to headless forms, like 

 Figs. 52 and 53. The headless forms produce all parts 

 of the body basal to the level which they represent, but 

 never give rise to any part characteristic of more apical 

 levels. The reason why they do not produce heads 

 will appear in the following section. Thus, headless 



1 Child, "Studies on the Dynamics of Morphogenesis, etc., I," 

 Jour, of Exp. Zool, X, 191 1 ; II, ibid., XI, 191 1 ; " Experimental Control 

 of Morphogenesis in the Regulation of Planaria," Biol. Bull, XX, 1911. 



