But is there no possibility of explaining the phenomena of organogenetic localisation by any other sort of interaction of parts? Two such possibilities may at the first glance seem to exist.
No Explanation Offered by a Chemical Theory of Morphogenesis
Though never set forth, in the form of a properly worked-out theory, the view has sometimes been advocated by biologists, that a chemical compound of a very high degree of complication might be the very basis of both development and inheritance, and that such a chemical compound by its disintegration might direct morphogenesis.
Let us first examine if such a view may hold for the most general features of organic morphogenesis. It seems to me that from the very beginning there exists one very serious objection to every chemical theory of form-building, in the mere fact of the possibility of the restoration of form starting from atypical localities. The mere fact, indeed, that there is such a thing as the regeneration of a leg of a newt—to say nothing about restitution of the harmonious type—simply contradicts,[62] it seems to me, the hypothesis, that chemical disintegration of one compound may govern the course of morphogenetic events: for whence comes the re-existence of the hypothetical compound, newly to be disintegrated, after disintegration has been completed once already? And we even know that regeneration may go on several times running from the same locality!
But, if we intentionally disregard this difficulty, in spite of its fundamental character, how could the hypothesis of chemical disintegration give the reason for the differentiation of our harmonious-equipotential systems, with special regard to the localisation of it; how could it account, in other words, for the appearance of typically localised specifications in an organ for which no external localising causes can be predicated?
Let us remember that a few original intimate differences exist in our harmonious systems: the main directions of the intimate protoplasmic structure including polarity and bilaterality. There are therefore three times two specified poles in each of these systems, at least in bilateral organisms, but no other differences are present in them. A few very simple cases of harmonious differentiation might indeed be understood on the theory of a disintegrating chemical compound in connection with these few differences. Imagine that the original compound, of the quantity a, is disintegrated to the amount of a1; from a1 are formed the two more simple compounds, b and c, both of them in definite quantities; then we have the three chemical individuals, a-a1, b and c, as the constituents of our harmonious system; and it now might be assumed, without any serious difficulty, though with the introduction of some new hypotheses, that the two poles of one of the fundamental axes of symmetry attract b and c respectively, a-a1 remaining unattracted between them. We thus should have the three elementary constituents of the system separated into three parts, and as they all three are of a definite quantity, their separation would mean that the system had been divided into three parts, a-a1, b and c, also with regard to its proper form. It is clear, that by taking away any part of the original system, by means of operations, there would be taken away a certain amount of the original compound; say that a/n is left; then, of course, the three constituents after the partial disintegration would be a-a1/n, b/n and c/n, and so it follows that the proportionality of localisation would really be preserved in any case.
But these considerations, evident as they seem to be in the most simple case, fail to satisfy in a really general sense: for two different reasons. First, they could never account for the fact that the differentiated organism by no means consists of so many different compounds as it shows single parts of its differentiation, but that, on the contrary, it only consists, as we know, of a certain rather limited number of true different morphogenetic elements, these elements occurring again and again—as for instance, nervous or muscular elements—but typical each time in locality, quantity, and form. And in the second place, the very form of elementary organs, their form as such, does not at all go hand-in-hand with chemical differences; this feature alone would absolutely overthrow any sort of a chemical morphogenetic theory to account for the problem of localisation. Take the typically arranged ring of the mesenchyme cells in our Echinus-gastrula, with its two spherical triangles, so typically localised; look at any sort of skeleton, in Radiolaria, or in starfishes, or in vertebrates: here you have form, real form, but form consisting of only one material. Not only is the arrangement of the elements of form typical here, e.g. the arrangement of the single parts of the skeleton of the hand or foot, but also the special form of each element is typical, e.g. the form of each single bone of the foot; and, on a purely chemical theory of morphogenesis the sufficient reason for the production of typical form in such a sense would be wanting. For atoms or molecules by themselves can only account for form which is arranged, so to speak, according to spatial geometry—as in fact they do in crystallography; but they can never account for form such as the skeleton of the nose, or hand, or foot. You will answer me perhaps, that there may be non-chemical agents in the germ,[63] responsible for typical form-localisation, but by such reasoning you would be departing from a purely chemical theory. Our next paragraph will be devoted to this side of the question.
That is the principal reason for rejecting all sorts of chemical morphogenetic theories put forward to explain the problem of localisation; it is more explicit, and therefore, I suppose, still more convincing than the more general consideration that the very fact of restitutions in itself must contradict the hypothesis that a disintegration of compounds might be the directive agency in morphogenesis. To sum up: Specificity of organic form does not go hand-in-hand with specificity of chemical composition, and therefore cannot depend on it; and besides that, specific organic form is such that it can never be explained by atomic or molecular arrangement in the chemical sense; for, to state it in a short but expressive manner, the “form” of an atom or molecule can never be that of a lion or a monkey. To assume that would be to go beyond the limits of chemistry in chemistry itself.