434 PATTERNS AND PROBLEMS OF DEVELOPMENT 



an external gradient ; in those cases dominance probably develops second- 

 arily from the high region. It appears sufficiently evident that physiologi- 

 cal and external factors effective in determining new developmental pat- 

 tern are, in general, factors influencing the physiological activity, the 

 metabohsm, of Hving protoplasm. Usually they are activating factors, 

 though in some cases pattern of one kind can be transformed into another 

 by depressing or inhibiting factors — for example, the transformation of 

 hydranth-stem axes into stolon axes by low oxygen tension or by inhibit- 

 ing chemical agents. In view of all the evidence developmental pattern 

 appears to be primarily an expression of the dynamics of hving proto- 

 plasms. 



