It has been known for many years that the Protozoa are also capable of a restoration of their form and organisation after disturbances, if at least they contain a certain amount of their nuclear substance. This process of restoration used to be regarded as belonging to the common type of regeneration proper, until T. H. Morgan succeeded in showing that in the genus *Stentor* it follows just the very lines which we know already from our study of embryonic organs or from *Tubularia*; that an harmonious-equipotential system is at the basis of what goes on. Now, you know that all Protozoa are but one highly organised cell: we have therefore here an instance where the so-called “elements” of our harmonious-morphogenetic system are not cells, but something inside of cells; and this feature must appear to be of very great moment, for it first shows, as we have already pointed out on another occasion, that morphogenesis is not dependent on cell-division, and it states at the same time that our concept of the harmonious-equipotential system may cover a very great area--that, in fact, it is a scheme of a very wide extent. *The Problem of the Factor* E We turn back again to considerations of a more abstract form. We left our analysis of the differentiation of the harmonious-equipotential systems, and particularly of the phenomena of localisation during this differentiation, at the point where we had succeeded in obtaining an equation as the expression of all those factors on which the prospective value, the actual fate, of any element of our systems depends, *p.v. (X) = f(s, l, E)* was the short expression of all the relations involved; *s* and *l*, the absolute size of the system and the relative position of the element with respect to some fixed points, were independent variables; *E* was a constant, namely, the prospective potency, with special regard to the proportions embraced by it. We shall now study the significance of the factor *E*. What does this *E* mean? Is it a short expression merely for an actual sum of elemental agents having a common resultant? And, if so, of what kind are these agents? Or what may *E* mean, if it can be shown *not* to be a short sign for a mere sum? *No Explanation Offered by “Means” or “Formative Stimuli”*