328 Action of the Genetic Material 



the head, kidneys in the rump, etc.). This may again be described in 

 terms of competence, established prior to the inductor action. Thus it 

 is clear that the inductor action does not replace the ordinary type of 

 genetic determination, but adds to it a specific feature. In a certain 

 sense we may compare this situation to that of control of sexual differ- 

 entiation both with and without hormones. In the latter case, an over- 

 all action by a relatively simple chemical substance is superimposed 

 upon the primary genie control of formation of the gonad, which pro- 

 duces the hormones. In the former case, differentiation is under simple 

 direct genie control — "simple" meaning that it occurs in the individual 

 cells after the manner of all genie actions. In the same way, amphibian 

 development is genically controlled by the interplay of genie material 

 and cytoplasmic substrate. But superimposed is the action of probably 

 a single chemical substance, not contained in a single gland but 

 present in all cells, which when released "orders" over-all happenings 

 of differentiation. We might express it by saying that the inductor 

 (X-substance) stimulates something to happen, but what happens is 

 decided by the same method as in non-inductive development. The 

 inductor action, like hormonic action, is a chemical method of integrat- 

 ing processes in many cells simultaneously, but not of determining 

 them. We might also express this by saying it is a technical advance 

 from the condition of the more or less mosaic development, simplffy- 

 ing and also unifying integration of basic developmental processes. 

 Thus the inductor type of development (with the many transitional 

 types found in insects by Seidel, 1936, and his students) does not 

 affect the views on genie cytoplasmic interaction developed previously. 

 But our way of looking at the facts is not the interpretation given 

 by embryologists like Brachet and Holtfreter. Holtfreter (1951) ex- 

 presses the relations between inductor and the progressively narrow- 

 ing competence of the embryonic cells thus. The state of competence 

 of the reactive material determines the direction of differentiation to 

 the same extent as does the specificity of the inductor. In the early 

 gastrula the ectoderm is pluripotent. Which one of these potencies 

 will be realized depends upon the surroundings, especially the adja- 

 cent inductor. With increasing age, the competence is narrowed, and 

 finally the ectoderm can no longer react to the stimulus of the induc- 

 tor. It is strange that the induction works only when it is needed, 

 though the capacity for induction remains in all tissues, dead or alive. 

 It is important that the action of the inductor, at its proper time, is 

 needed only for a short time; afterward, differentiation goes on under 

 its own steam. But some parts of the embryo — and this is important — 



