THE MUTATED GENE 201 



of Driesch, whence its prospective potency is identical with its 

 prospective fate. The processes that lead to this type of pattern 

 formation are usually conceived as being of two types, self- 

 differentiation and dependent, or inductive, differentiation. 

 But it may be regarded as certain now that these two types of 

 developmental activity are, in fact, only two phases of the same 

 process. If we try to describe an inductive process in a most 

 general way, it would be one that leads to pattern-like restriction 

 of potencies in an area outside the area containing the inductive 

 agent. Induction of pattern by a process or product of a process 

 within the area to be subdivided would also be induction (see 

 page 202). It would then be an inductive action if a hormone 

 were sent to the area of induction or if a substance formed in the 

 inductor were to diffuse into the area to be induced or if a con- 

 tact between two areas, if established, were to produce an 

 unknown physical effect, followed by pattern formation. 1 



The best known inductive agency is the organizing center of 

 amphibians. We know that its active substance, at least for 

 the general inductive effect, is a relatively simple chemical com- 

 pound (though unanimity is not yet established regarding its 

 actual nature); therefore the inducing action must be conceived 

 as diffusion of this substance into another area. This process, 

 then, would be comparable to the determination stream already 

 mentioned, though not completely. Furthermore, we know that 

 there is a regional differentiation within the inductor area, which 

 has different inductive effects, e.g., a part that may induce only 

 head pattern. This shows, then, the presence of a pattern within 

 the inductive area, whatever it is. An inductor, then, is some- 

 thing rather general (not very specific) ; something that may show 

 a certain diversity within this generalized nature; something that 

 is moved to adjacent areas; something that makes the area upon 

 which it acts diversify, form a pattern, the details of which 

 depend upon the constitution of the inductor substance and upon 

 the condition of the area to be induced. Finally, the first set of 

 inductors may be followed by a second or secondary one within 



controlled by genes, and also final in the average type of experimentation, 

 we feel entitled to use this conception. 



1 The general literature on experimental embryology will not be quoted 

 in this section, except when special examples are used in connection with the 

 topic of this book. 



