Thus we arrive in this way also at our earlier assumption that an inactive accessory-idioplasm is given to all, or at least to many cell-generations. Only among plants must this necessarily be complete germ-plasm, and among the lower plant-forms, as in Caulerpa among the Algæ, in Marchantia among Liverworts, it must be assumed to be present in nearly all the cells, according to the experiments in regeneration made by Reinke and Vöchting. But in multicellular animals which develop from two different germinal layers equipped with a different complex of determinants budding arises from a combination of at least two different kinds of cells, and we must only ascribe to each of these its own peculiar determinant-complex as regeneration-idioplasm. Higher plants show us that well-marked power of budding is not necessarily associated with a high regenerative capacity, the histologically specialized cells among them will contain no inactive germ-plasm, because they do not need it. But in animals the power of budding is probably always combined with high regenerative capacity, as is shown by the Polyps and Medusoids above all, and in a different way by the Ctenophores, which exhibit no budding and at the same time a very slight regenerative capacity, although they possess an organization scarcely higher than that of the Hydromedusæ. In the Ctenophores each of the first segmentation-cells, when artificially separated, yields only a half-embryo, and we may conclude from this that it contains no complete germ-plasm in an inactive state, or at least very little, and certainly not a sufficient quantity to make it readily regenerative.
Undoubtedly, however, the regenerative capacity occurs apart from the capacity for budding, yet this in no way contradicts the theory. As we have seen, a high regenerative capacity is to be found among many animals which occur only as 'persons' and not as colonies or stocks, but only in those which are readily liable to injury, and only in the manner conditioned by their injury. In the higher Metazoa the regenerative power becomes more and more limited, and in the Mammals it sinks to a mere closing up of wounds.
If we take a survey of the assumptions we have been compelled to make from the standpoint of the theory to explain the development of germ-cells, budding, and regeneration, it would seem as if it were contradictory to assume that, on the one hand, complete germ-plasm should be given to certain cell-series as inactive accessory idioplasm, and, on the other, that very numerous cells, at least in the lower Metazoa, should have received the idioplasm of budding, and still more numerous cells that of regeneration. But it is obvious that among the lower Metazoa the idioplasm of budding and the idioplasm of regeneration are equivalent; the same idioplasm, which, when liberated by stimuli unknown to us, co-operates from two or three germinal layers in the formation of a bud, effects, in response to the known stimulus of injury, the regeneration of the mutilated part. But germ-cells can never arise in the Metazoa from the partial budding-idioplasm or regeneration-idioplasm, because this is not complete germ-plasm, and because it can only give rise to budding or regeneration through the co-operation of two or more kinds of cells, while germ-cells always originate from one cell and never arise from the fusion of cells. Germ-cells can thus only arise from the cells of the germ-track, and in no other way, no matter whether the germ-track lie in the ectoderm, as in the Hydromedusæ, or in the endoderm, as in true jellyfishes (Acalephæ) and the Ctenophores, or in the mesoderm, as in many higher groups of animals. It is only apparently that these cells belong to one particular layer, for in reality they are unique in kind, and they are simply assisted in their development by one or other cell-layer, from which they not infrequently emancipate themselves, as happens so notably in the Hydromedusæ. As we have already said, it is only among plants that we must think of budding as arising from cells which contain complete germ-plasm, for here there are no 'germinal layers' corresponding to those of animal development, and the cells of 'the growing point' must be equipped with the complete germ-plasm. The plant, like the Hydroid stock and the Siphonophore colony, is saved from death, in spite of the frequent loss of its members, mainly by the fact that it is capable of producing, at almost any part above the ground, buds which develop into new shoots, with leaves and the like. This makes a power of regeneration on the part of the individual leaves and flower-parts superfluous, but at the same time it implies that an enormous number of cells must be distributed over the whole surface of the plant, each of which can in certain circumstances become the starting-point of a bud. That is to say, each must contain, in a latent state, the complete germ-plasm which is necessary for the production of an entire plant.
We must therefore assume that, in the higher colony-forming plants, germ-plasm is contained in a great many cells, perhaps in all which are not histologically differentiated, and sometimes even in those which are so, as, for instance, in the leaves of Begonias. I suppose, therefore, that in the higher plants the process of development implies a segregation of the determinant-complexes of the germ-plasm, but that this takes place at a late stage, and that in a much higher degree than among animals the individual or the 'person' carries with it germ-plasm in a latent state. To this must be attributed the fact that the plant is not only able to make good its losses in twigs and branches by sending out new shoots, but that cuttings, that is, detached shoots, are also able to take root, and in general to give rise to what is necessary to complete themselves according to the position of the part in question. In the ontogeny of animals, too, we must assume that it requires a liberating stimulus to rouse the determinants to activity, that this stimulus is to be sought for in the influence exercised by the constitution of the cell on the idioplasm contained within it, and that this constitution in its turn is subject to influences from external conditions, including the cell-soma itself. We may therefore suppose that, among plants also, the germ-plasm latent in numerous cells only becomes active in whole or in part according to the influences exerted on it by the state of the cell at the moment; but this varies with external circumstances, according to whether the cell is exposed to light or lies under ground, according as it is influenced by gravity, by moisture, chemical stimuli, and so on.
It might be objected to this that it would be simpler not to assume a segregation of the germ-plasm into determinant-complexes at all in order to explain the process of development, but rather to credit each cell with a complete equipment of germ-plasm from the beginning to the end of the ontogeny, and to attribute the differences in the cells, which condition the structure of the plant and its differentiation, solely to the different influences, external and internal, to which the cell is exposed, and which rouse some determinants to activity at one part and others at another. Perhaps the botanists would be more readily reconciled to this idea, but it seems to me that there are two points which tell against the possibility of its being correct. In the first place, it is far from being established that every cell in the higher plants is capable of giving rise, under favourable conditions, to a whole new plant; every tree and every higher plant has a multitude of cells in its leaves, its flowers, and so on, which cannot do this, which are in fact differentiated in one particular direction, that is, they contain only one kind of determinants, like the histologically differentiated cells of the tissues of the human body. Secondly, there are other organisms besides plants, and a theory of development cannot be based on the phenomena to be observed among plants alone, any more than a theory of heredity can. There are obvious differences in the processes of life among plants as contrasted with those among animals, but it is improbable that there is any thoroughly fundamental difference. It is, however, indubitable that the cells forming the tissues of higher animals, the nerve, muscle, and glandular cells, are really differentiated in one direction, and are quite incapable, under any circumstances whatever, of growing into an entire organism, and even from this alone we might conclude that they contain only one primordium or determinant. Are we then to assume that the vascular cells, epidermis-cells, wood-cells, and so on, of the higher plants, which are also differentiated in one direction, do nevertheless contain the complete germ-plasm? I do not see any ground for such an assumption.
To conclude what can be said on the subject of regeneration we must return to the question of an ultimate explanation of this marvellous phenomenon. I have declined to attempt any explanation at all, because I do not consider it possible to give a sufficient one as yet, but I should like at least to give an indication as to the direction in which we must look for it.
We assumed that there is a regeneration-idioplasm, and therefore that there are 'primary constituents' at certain positions in the body, but how does it happen that these are able to build up the lost parts in the proper situation and detail? A theoretical formula might well be thought out, according to which the determinants of successive parts would become active successively, and would thus liberate one another in an appropriate order of sequence, but there would not be much gained by this, especially as what we already know in regard to the regrowth of the legs and toes in Triton does not harmonize with such an assumption. It appears to me more important—though even here we must still be very vague as to details—to recognize that, in all vital units, there are forces at work which we do not yet know clearly, which bind the parts of each unit to one another in a particular order and relation. We were obliged to assume such forces even in regard to the lowest units, the biophors, since otherwise they could not be capable of multiplication by division, on which all organic growth depends, unless we are to assume, as Nägeli did, a continual generatio æquivoca of the specific kinds of biophors (his 'micellæ'). But we shall see later, when we come to speak of spontaneous generation, that we cannot acquiesce in such an assumption. If, then, we cannot conceive of a power of division arising from within and depending solely on growth by means of assimilation, without such attractive and repellent forces or 'vital affinities' the internal parts would necessarily fall into disorder at every division. It seems to me therefore that such 'affinities' must be operative at all stages in the life of the vital units, not only in biophors, but also in the cell, and in the 'person' as well as in determinant and id. It is true that 'persons' no longer generally possess the power of multiplying by division, but in plants and lower animals many do possess it; and the power of giving rise anew to certain parts is obviously a part of that power of doubling the whole by division. The ultimate roots of regeneration, then, must lie in these 'affinities' between the parts, which preside over their arrangement and are able to maintain it and to give rise to it anew. In this respect the organism appears to us like a crystal whose broken points always complete themselves again from the mother-lye after the same system of crystallization, obviously in this case too as a result of certain internal directive forces, polarities, which here again we are unable precisely to define. But the difference between the organism and the crystal does not—as people have been hitherto inclined to believe—lie only in the fact that the crystal requires the mother-lye to complete itself, while the vital unit itself procures the material for its further growth; it lies also in the fact that such regeneration is not possible in every organism and at every place, but that special 'primary constituents' are necessary, without which the relevant part cannot arise. The indispensableness of these primary constituents, the determinants, seems to me to depend on the fact that the new structure cannot be built up simply by procuring organic material, but that specially hewn stones, different in every case, are necessary, which can only be supplied in virtue of an historical transmission, or, to abandon the metaphor, because the vital units of which the organ is to be reconstructed possess a specific character and have a long history behind them; thus they can only arise from such vital units as have been handed on through generations, that is, from the determinants. But these primary constituents are given to the different forms of life in very varying degrees and in very unequal distribution, and as far as we can see according to their suitability to an end.