Let us begin by laying down two fundamental concepts. Suppose we have here a definite embryo in a definite state of development, say a blastula, or a gastrula, or some sort of larva, then we are entitled to study any special element of any special elementary organ of this germ with respect to what is actually to develop out of this very element in the future actual course of this development, whether it be undisturbed or disturbed in any way; it is, so to say, the actual, the real fate of our element, that we take in account. I have proposed to call this real fate of each embryonic part in this very definite line of morphogenesis its prospective value (“prospective Bedeutung” in German). The fundamental question of the first chapter of our analytical theory of development may now be stated as follows: Is the prospective value of each part of any state of the morphogenetic line constant, i.e. is it unchangeable, can it be nothing but one; or is it variable, may it change according to different circumstances?
We first introduce a second concept: the term prospective potency (“prospective Potenz” in German) of each embryonic element. The term “prospective morphogenetic potency” is to signify the possible fate of each of those elements. With the aid of our two artificial concepts we are now able to formulate our introductory question thus: Is the prospective potency of each embryonic part fully given by its prospective value in a certain definite case; is it, so to say, identical with it, or does the prospective potency contain more than the prospective value of an element in a certain case reveals?
We know already from our historical sketch that the latter is true: that the actual fate of a part need not be identical with its possible fate, at least in many cases; that the potency of the first four blastomeres of the egg of the sea-urchin, for instance, has a far wider range than is shown by what each of them actually performs in even this ontogeny. There are more morphogenetic possibilities contained in each embryonic part than are actually realised in a special morphogenetic case.
As the most important special morphogenetic case is, of course, the so-called “normal” one, we can also express our formula in terms of special reference to it: there are more morphogenetic possibilities in each part than the observation of the normal development can reveal. Thus we have at once justified the application of analytical experiment to morphogenesis, and have stated its most important results.
As the introductory experiments about “Entwickelungsmechanik” have shown already that the prospective potency of embryonic parts, at least in certain cases, can exceed their prospective value—that, at least in certain cases, it can be different from it—the concept of prospective potency at the very beginning of our studies puts itself in the centre of analytical interest, leaving to the concept of prospective value the second place only. For that each embryonic part actually has a certain prospective value, a specified actual fate in every single case of ontogeny, is clear from itself and does not affirm more than the reality of morphogenetic cases in general; but that the prospective value of the elements may change, that there is a morphogenetic power in them, which contains more than actuality; in other words, that the term “prospective potency” has not only a logical but a factual interest: all these points amount to a statement not only of the most fundamental introductory results but also of the actual problems of the physiology of form.
If at each point of the germ something else can be formed than actually is formed, why then does there happen in each case just what happens and nothing else? In these words indeed we may state the chief problem of our science, at least after the fundamental relation of the superiority of prospective potency to prospective value has been generally shown.
We consequently may shortly formulate our first problem as the question of the distribution of the prospective morphogenetic potencies in the germ. Now this general question involves a number of particular ones. Up to what stage, if at all, is there an absolutely equal distribution of the potencies over all the elements of the germ? When such an equal distribution has ceased to exist at a certain stage, what are then the relations between the parts of different potency? How, on the other hand, does a newly arisen, more specialised sort of potency behave with regard to the original general potency, and what about the distribution of the more restricted potency?
I know very well that all such questions will seem to you a little formal, and, so to say, academical at the outset. We shall not fail to attach to them very concrete meanings.