Our plane of the dimensions *a* and *b* is the basis of the normal, undisturbed development; taking the sides of the plane as fixed localities for orientation, we can say that the actual fate, the “prospective value” of every element of the plane stands in a fixed and definite correlation to the length of two lines, drawn at right angles to the bordering lines of the plane; or, to speak analytically, there is a definite actual fate corresponding to each possible value of *x* and of *y*. Now, we have been able to state by our experimental work, that the prospective value of the elements of our embryonic organ is not identical with their “prospective potency,” or their possible fate, this potency being very much richer in content than is shown by a single case of ontogeny. What will be the analytical expression of such a relation? Let us put the question in the following way: on what factors does the fate of any element of our system depend in all possible cases of development obtainable by means of operations? We may express our results in the form of an equation:-- *p.v. (X) = f( ... )* *i.e.* “the prospective value of the element *X* is a function of ...”--of what? We know that we may take off any part of the whole, as to quantity, and that a proportionate embryo will result, unless the part removed is of a very large size. This means that the prospective value of any element certainly depends on, certainly is a function of, the *absolute size* of the actually existing part of our system in the particular case. Let *s* be the absolute size of the system in any actual experimental case of morphogenesis: then we may write *p.v. (X) = f(s ... )*. But we shall have to add still some other letter to this *s*.