It might be supposed that the structure of these biophors corresponded in advance to certain constituent parts of the cell, that there were, for instance, muscle biophors, which make the muscle what it is, or that the plant-cells acquired their chlorophyll-making organs through chlorophyll biophors. De Vries gave expression to this view in his 'pangen theory,' and I confess that at the time there seemed to me much to be said for it, but I am now doubtful whether its general applicability can be admitted. In the first place, it does not seem to me theoretically necessary to assume that the particles which migrate into the cell-bodies should themselves be chlorophyll or muscle particles; they may quite well be only the architects of these, that is to say, particles which by their co-operation with the elements already present in the cell-body give rise to chlorophyll or muscle substance. As we are as yet unacquainted with the forces which dominate these smallest vital particles, as well as the processes which lead to the histological differentiation of the cells, it is useless in the meantime to make any further hypotheses in regard to them. But in any case the biophors which transform the general character of the embryonic cells into the specific character of a particular tissue-cell must themselves possess a specific structure different from that of other biophors, for they must keep up the continuity of the structures handed on from ancestors, chlorophyll and muscle-substance and the like, since we cannot assume that these structures, so peculiar and so complex in their chemical and physical constitution, are formed afresh, so to speak, by spontaneous generation in each new being, as De Vries has very rightly emphasized. A specific biophor, for instance, of muscle substance will produce this substance as soon as it has found its way into the appropriate cell-body, even though it may not be a contractile element itself.
To this must be added that the structure of the body and the distinctive features of an organism do not depend merely on the histological differentiation of the cells, but quite as much on their number and arrangement, and on the size and on the frequency of repetition of certain parts. These distinctive characters are just as constant and as strictly transmissible, and may be as heritably variable as those which depend on specific cell-differentiation, and they must therefore likewise be determinable by definite elements of the germ-plasm. Obviously enough, however, these elements are not of the same nature as the known specific histological elementary particles; they can be neither nerve-, muscle-, nor gland-biophors. They must rather be vital units of such a kind that they communicate to the cells and lineage of cells, into whose bodies they migrate from within the nucleus, a definite vital power, that is, an organization which regulates the size, form, number of divisions, and so on, of these cells—in short their whole prospective significance. Always, however, they act in co-operation with the cell-body into which they have penetrated.
Throughout we must hold ourselves aloof from the idea that 'characters' are transmissible. It is customary, indeed, to speak as if this were so, and it is also necessary, because we can only recognize the 'characters' of a body, and not the essential 'nature' on which these characters depend; but the determinants are not seed-grains of individual characters, but co-determinants of the nature of the parts which they influence. There are not special determinants of the size of a cell, others of its specific histological differentiation, and still others of its duration of life, power of multiplication, and so on; there are only determinants of the whole physiological nature of a cell, on which all these and many other 'characters' depend. For this reason alone I should object to the assumption that the determinants of the germ are ready-made histological substances. That is as unlikely as that their groups in the germ-plasm are 'miniature models' of the finished parts of the body.
I conceive of the process of cell-differentiation as follows: at every cell-stage in the ontogeny determinants attain to maturity, and break up so that their biophors can migrate into the cell-bodies, so that the quality of each cell is thus kept continually under control, and may be more or less modified, or may remain the same. By the 'maturity' of a determinant I mean its condition when by continual division it has increased in number to such a point that its disintegration into biophors and their migration into the cell-substance can take place.
One more point I must touch upon here, the question of the 'liberation' or 'stimulation' of the determinants. The activity of an organ never depends on itself alone; the contraction of a muscle is induced by a nerve stimulus or by an electric current; the activity of the nerve-cells of the brain requires the continual stimulus of the blood-stream, and cannot continue to exist without it; the specific sensory-nerves and sense-cells of the eye, ear, olfactory organ, and so on, are all prompted to activity by adequate stimuli. The same is true in regard to the determinants, they must be 'liberated' if they are to distribute themselves and migrate into the cell-body; and we have to ask how that happens, whether it is possibly due only to their own internal condition, which again would, of course, depend on the nutritive conditions of the cell in which they lie, or whether it is perhaps due to some specific stimulus which is necessary in addition to the fact of 'maturity,' just as a muscle is always ready to contract, yet only does so when it is affected by a specific stimulus.
From the very first, therefore, I have considered whether it would not be better to elaborate the determinant theory in such a way that it would not be necessary to assume a disintegration of the id in the course of ontogeny, but simply to conceive of every expression of activity on the part of a determinant as dependent on a specific stimulus, which in many cases can only be supplied by a definite cell, that is, by internal influences, and in other cases may be due to external influences.
Darwin assumed the first of these alternatives in his theory of Pangenesis, which we have still to outline. In it he attributes to his 'gemmules' the power of giving rise to particular cells, which, however, they can only accomplish when they reach the cells which are the genetic antecedents of those which the gemmules are to control. Translated into the language of our theory this view would read as follows: the whole complex of determinants is contained within every cell, as it is contained in the germ-cell, but at every stage of ontogeny, that is, in each of the developing cells, only the determinant which is to control the immediately successive cells is 'liberated,' and that through the stimulus which the specific nature of the cell supplies to the determinant. In that case there would necessarily be in every species of animal as many specific stimuli for determinants as there are determinants. This appeared to me improbable, and I rejected the hypothesis because of the enormous number of specific stimuli which it demands, but also on other grounds which will be touched upon in the course of these lectures.
Although the assumption of an autonomic dissolution of the determinant complexes of the id in the course of ontogeny seems to me imperative, I do not by any means reject the interposition of liberating stimuli, indeed I regard their co-operation as indispensable. Later on we shall discuss cases in which it is definitely demonstrable that there may be two alternative sets of homologous determinants present in a cell, but that on any occasion only one of these becomes active, a fact which we can only explain on the assumption that only one of these is affected by the specific liberating stimulus. The phenomena of regeneration, of polymorphism, of germ-cell formation, &c., compel us to the assumption that numerous cells, even after the completion of the building up of the body, contain two or more kinds of determinants, as in a sense inactive 'accessory idioplasm,' each of which could control the cell alone, though in reality it only does control it when it is affected by the appropriate liberating stimulus. I stated this view some years ago when I attempted to define more precisely the rôle played by 'external influences as developmental stimuli[21]'. It is not, then, that I underrate the importance of external influences on the organism, but I believe that a still larger part of the determination of what shall happen at a particular point depends on the primary constituents, and that these are not alike at all parts of the body.
[21] Äussere Einflüsse als Entwicklungsreize [External Influences as Stimuli to Development], Jena, 1894.
All living processes, therefore, both those of growing and of differentiation, depend always upon the interaction of external and internal factors, of the environment and the living substance, and the resultants of the interaction, namely, the structure of the body and its parts must necessarily turn out differently, not only when the germ-substance is different, but when the essential conditions of development are changed. But that the constitution of the germ is by far the most potent factor, and that the nature of the results of development depends on it in a much greater degree than on the external conditions, has long been known. The conditions, such as warmth, may vary within certain limits, and yet the frog's egg becomes a frog; though it does not follow that the result of development may not be modified through certain changes in the conditions. The interesting experiments made by Herbst with the eggs of sea-urchins have shown that, in artificially altered sea-water in which sodium-salts are to a slight extent replaced by lithium-salts, these eggs develop into larvæ which only remotely suggest the normal structure, and diverge widely from it both in external shape and in the form of the skeleton.