But a further experiment has been made on our gastrula. If at the moment when the material of the future intestine is most distinctly marked in the blastoderm, but not yet grown into a tube, if at this moment the upper half of the larva is separated from the lower by an equatorial section, you will get a complete larva only from that part which bears the “Anlage” of the endoderm, while the other half will proceed in morphogenesis very well but will form only ectodermal organs. By another sort of experiment, which we cannot fully explain here, it has been shown that the endoderm if isolated is also only able to form such organs as are normally derived from it.
And so we may summarise both our last results by
saying: though ectoderm and endoderm have their potencies equally distributed amongst their respective cells, they possess different potencies compared one with the other. And the same relation is found to hold for all cases of what we call elementary organs: they are “equipotential,” as we may say, in themselves, but of different potencies compared with each other.
Explicit and Implicit Potencies: Primary and Secondary Potencies
We shall first give to our concept of “prospective potency” a few words of further analytical explanation with the help of our newly obtained knowledge.
It is clear from what we have stated that the prospective potencies of the ectoderm and of the endoderm, and we may add, of every elementary organ in relation to every other, differ between themselves and also in comparison with the blastoderm, from which they have originated. But the diversity of the endoderm with respect to the ectoderm is not of the same kind as its diversity in respect to the blastoderm. The potency of the endoderm and that of the ectoderm are both specialised in their typical manner, but compared with the potency of the blastoderm they may be said not only to be specialised but also to be restricted: the potency of the blastoderm embraces the whole, that of the so-called germ-layer embraces only part of the whole; and this species of restriction becomes clearer and clearer the further ontogeny advances: at the end of it in the “ultimate elementary organs” there is no prospective potency whatever.
A few new terms will serve to state a little more accurately what happens. Of course, with regard to all morphogenesis which goes on immediately from the blastoderm, the potency of the blastoderm is restricted as much as are the potencies of the germ layers. We shall call this sort of immediate potency explicit, and then we see at once that, with regard to their explicit potencies, there are only differences among the prospective potencies of the elementary organs; but with respect to the implicit potency of any of these organs, that is with respect to their potency as embracing the faculties of all their derivations, there are also not only differences but true morphogenetic restrictions lying at the very foundations of all embryology.
But now those of you who are familiar with morphogenetic facts will object to me, that what we have stated about all sorts of restrictions in ontogeny is not true, and you will censure me for having overlooked regeneration, adventitious budding, and so on. To some extent the criticism would be right, but I am not going to recant; I shall only introduce another new concept. We are dealing only with primary potencies in our present considerations, i.e. with potencies which lie at the root of true embryology, not with those serving to regulate disturbances of the organisation. It is true, we have in some way disturbed the development of our sea-urchin’s egg in order to study it; more than that, it would have been impossible to study it at all without some sort of disturbance, without some sort of operation. But, nevertheless, no potencies of what may properly be called the secondary or restitutive type have been aroused by our operations; nothing happened except on the usual lines of organogenesis. It is true, some sort of regulation occurred, but that is included among the factors of ontogeny proper.
We shall afterwards study more fully and from a more general point of view this very important feature of “primary regulation” in its contrast to “secondary regulation” phenomena. At present it must be enough to say that in speaking of the restriction of the implicit potencies in form-building we refer only to potencies of the primary type, which contain within themselves some properties of a (primary) regulative character.
The Morphogenetic Function of Maturation in the Light of Recent Discoveries