The conception of “prospective potency” was defined by its author H.Driesch as “the sum of the possible modes of development of a given part of the embryo”. However, in the language of science the word “potency” usually means “power”, not “possibility”. Therefore this conception from the beginning possessed a certain ambiguity, that it retained up to the present. The distinction of “active” and “passive” potency, introduced by Roux, has found no general acceptance. It appeared necessary to submit the conception of (...) “potency” to a profound analysis. In the first place it is shown, that the ambiguity of this conception vanishes, if we understand by the word “potency” those fundamental properties of the embryonic material, that are responsible for its various developmental possibilities. We can distinguish among these properties the “reactive powers”, by which the embryonic material reacts to inductive stimulations, and its own “developmental tendencies”. These faculties only differ by their strength; a “reactive power” may change into a “tendency”, when its intensity increases during development; conversely, a “tendency” may change into a mere “reactive power” by a decrease of its strength. The determination of the material results from the conflict of its tendencies. The inductive actions interfere by strengthening or weakening different tendencies, and by activating new tendencies by reinforcement of the “reactive powers” of the embryonic material. (shrink)