No. 396.] REVIEWS OF RECENT LITERATURE. 969 
Another well-marked case in which this same phenomenon is mani- 
fest is the “ reparation ” of Tubularia. How is it that the two circlets 
of buds of the new hydranth arise not on the surface of the wound 
of the transsected hydrocaulus, but always at a definite distance from 
that surface ? 
Finally, the results of Morgan’s experiments upon regeneration in 
Planaria, which show that each of the several pieces cut from one 
individual becomes transformed by a shifting and differentiation of 
the protoplasm into a small and typically proportioned worm, likewise 
demand for an interpretation some controlling factor with localizing 
power. 
In a discussion of the idea of localization in general, it is shown 
that the phenomenon of determination of position of parts may or 
may not require for its explanation the postulate of a localizing force 
of anelementary kind. In many instances of localization phenomena 
among plants, as in the transformation of indifferent leaf germs into 
foliage leaves or bud scales by the influence or absence of light, the 
localizing power consists in the direct action of a definite external 
stimulus upon parts which possess equal, though restricted, possibil- 
ities. In such cases as this, and in ordinary regeneration among 
animals, like the restoration of an amputated leg in Triton, localiza- 
tion is determined by the nature and position of the external stimulus 
(light, heat, etc.) or by the limited “ potency” of the parts. There- 
fore no localizing action of an elementary nature occurs in these 
instances. 
It is shown that the localization phenomena which are independent 
of specific external stimuli and of a simple elementary nature are 
confined to that association of cells or elementary- structures of 
whatever kind which Driesch terms a harmonic equipotential system. 
In an equipotential system, in general, every part has an equal “ pro- 
spective potency” with every other, że., equal power as to the possi- 
bilities of its development. The willow twig, for example, is an 
equipotential system in which every part has indeed an equal pro- 
spective potency with every other; but here the power is limited to 
the production of sprouts and roots that are indefinite in number and 
have no specific relation to one another. An equipotential system 
which is thus restricted in its possibilities is called a determined 
equipotential system. In the segmenting egg of an echinoid or in 
the stem of Tubularia, however, every ¢ effect” appears only once 
or a limited number of times, and stands in a definite relation to all 
other “effects.” Such are accordingly called harmonic equipotential 
