GRADIENTS, FIELDS, AND DETERMINATION 291 



limb field is not equipotential at any given moment. Any other area in 

 it than the presumptive limb area must undergo change to become ca- 

 pable of giving rise to a limb, and hmbs from some parts of it may be 

 less complete than those from other parts. In short, for every particular 

 development the system, whether Tiibularia stem or limb field, must be- 

 come another system; since this is possible, it is equipotential in one sense, 

 and since it has a pattern, or a new one is produced in it, it is harmonious. 

 According to this view, the harmonious-equipotential system appears to 

 be a gradient system in which the cUfferent levels have not become so far 

 specifically different that they cannot react to altered conditions with an 

 altered development, that is, a system in which a similar pattern may be 

 differently localized under different conditions. In these terms it is far 

 from constituting proof of the autonomy of vital processes; but if organis- 

 mic pattern consists primarily in definitely locahzed specificities, as 

 Driesch assumed, it becomes difficult to see how a part can become a 

 whole without the aid of Driesch's entelechy or some other equally capable 

 metaphysical agent. As a matter of fact, various evidences that inequi- 

 potentiahties appear in systems assumed to be equipotential are found in 

 Driesch's data; but either they were ignored, or in certain cases it was 

 maintained that they represented results of mere physicochemical con- 

 ditions without the controlling action of entelechy. 



DETERMINATION AND DIFFERENTIATION 



In experimental analysis of development the terms "determine," "de- 

 termination," and "differentiation" are so generally used that some con- 

 sideration of their usage and the basis for it seems necessary. To deter- 

 mine experimentally a certain developmental result is to provide the con- 

 ditions necessary for it. We can determine certain differential modifica- 

 tions of pattern by exposure of the entire developing organism to toxic 

 agents (chaps, v-vii). New polarities and symmetries can be determined 

 in various ways (see. chap. xi). In recent years, however, we have come 

 to speak of determination of regions or parts in the course of development. 

 A part not yet visibly differentiated, that is, not yet morphologically 

 characterized, is regarded as determined when its development continues 

 unaltered — at least for some time — after change in, or isolation from, 

 organismic environment. Such a part is said to be capable of self-differ- 

 entiation. The change in environment may consist in transplantation to 

 another region of the same individual or to other individuals of the same 

 or other species; that is, it may be autoplastic, homoplastic, heteroplastic, 



