The operation of section was without restriction either as to the amount of the material removed from the germ, or as to the direction of the cut. Of course, in almost every actual case there will be both a definite size of the actual system and a definite direction of the cut going hand-in-hand. But in order to study independently the importance of the variable direction alone, let us imagine that we have isolated at one time that part of our system which is bounded by the lines *a_1 b_1*, and at another time an equal amount of it which has the lines *a_2 b_2* as its boundaries. Now since in both cases a typical small organism may result on development, we see that, in spite of their equal size the prospective value of every element of the two pieces cut out of the germ may vary even in relation to the direction of the cut itself. Our element, *X*, may belong to both of these pieces of the same size: its actual fate nevertheless will be different. Analytically, it may be said to change in correspondence to the actual position of the actual boundary lines of the piece itself with regard to the fundamental lines of orientation, *a* and *b*; let this actual position be expressed by the letter *l*, *l* marking the distance of one[58] of the actual boundary lines of our piece from *a* or *b*: then we are entitled to improve our formula by writing *p.v. (X) = f(s, l ... )* (Fig. 11). [58] The distance of the other boundary line from *a* or *b* would be given by the value of *s*. [Illustration: Fig. 11.--Diagram to show the Characteristics of an “Harmonious-equipotential System.” The element *X* forms part of the systems *a b* or *a_1 b_1* or *a_2 b_2*; its prospective value is different in each case.] But the formula is not yet complete: *s* and *l* are what the mathematicians call variables: they may have any actual value and there will always be a definite value of *p.v.*, *i.e.* of the actual fate which is being considered; to every value of *s* and *l*, which as we know are independent of each other, there corresponds a definite value of the actual prospectivity. Now, of course, there is also a certain factor at work in every actual case of experimental or normal development, which is *not* a variable, but which is the same in all cases. This factor is a something embraced in the prospective potency of our system, though not properly identical with it.