We have endeavoured to explain the handing on of the complement of heritable qualities from one generation to another as due to a continuity of the germ-plasm, and we assumed that the germ-cells never arise except from cells in the 'germ-track'; that is, from cells which are equipped, from the fertilized egg-cell onwards, with a complete sample of slumbering germ-plasm, and are thereby enabled to become germ-cells, and, subsequently, new individuals, in which the aggregate of inherited primary constituents implied in the germ-plasm can again attain to development.
We have now to consider other cases of inheritance in relation to the same problem—the origin of their hereditary equipment.
We know, of course, that new individuals may arise apart from germ-cells, that, in many of the lower animals and in plants, they may arise by budding and fission.
For both these cases the germ-plasm theory will suffice, with a somewhat modified form of the same assumption which we made in regard to the formation of germ-cells. The origin of a new individual by budding seems often, indeed, to proceed from any set of somatic cells in the mother animal; but somatic cells, if they contain solely the determinants controlling themselves, cannot possibly give rise to a complete new individual, since this presupposes the presence of all the determinants of the species. But as these determinants cannot be formed de novo, the budding cells must contain in addition to the usual controlling somatic determinants, idioplasm in a latent, inactive state, which only becomes active under certain internal or external influences, and then gives rise to the formation of a bud. The source of this accessory idioplasm must, however, be looked for only in the egg-cell.
In plants this bud-idioplasm must be complete germ-plasm, because the budding starts only from one kind of cell, the cambium-cells; but in animals in which—as it seems—it always proceeds from at least two different kinds of cells—those of the ectoderm and those of the endoderm—the matter is more complex. In this case these two kinds of cells will contain as bud-idioplasm two different groups of determinants, which mutually complete each other and form perfect germ-plasm, and only the co-operation of these two sets will give rise to the formation of a bud. I will not, however, go further into detail in regard to these relations, for the theory can do nothing more here than formulate what has been observed; it is hardly in a position to help us to a better understanding of the facts.
The case is not much clearer in regard to the processes which lead to the replacing of lost parts. The manifold phenomena of regeneration can also be brought into harmony with the theory, if we attribute to those cells from which the replacing or entire reconstruction of the lost part arises an 'accessory-idioplasm,' which, at least, contains the determinants indispensable to the building up of the part. It is possible that the assumed accessory idioplasm frequently contains a much larger complex of determinants, and that it depends on the liberating stimuli which, and how many of these, will become active.
If we take a survey of regenerative phenomena in the animal kingdom, it strikes us at once that the capacity is very different in different species, extraordinarily great in some and very slight in others. In general it is greater in lower animals than in higher, but, nevertheless, the degree of differentiation cannot be the only factor that determines the capacity for regeneration. That unicellular organisms can completely replace lost parts, that even a piece of an infusorian can reconstruct the whole animal if only the piece contain a part of the nucleus, we have already seen when discussing the significance of the nuclear substance. In this case the nucleus must contain the complete germ-plasm, that is, the collective determinants of the species, and these induce the reconstruction of the lost part, though they do so in a way that is still entirely obscure to us. In the meantime, our interpretation will not carry us further, either here or in regard to any other order of vital phenomena. To go further would be little short of propounding a causal theory of life itself; it would mean having a complete and real 'explanation' of what 'life' is. As yet no one has been able to claim this position. We can see the different stages through which every organism passes, and that they arise one out of the other; we can even penetrate down to the succession of those delicate and marvellously complex processes which effect nuclear and cell-division; but we are still far from being able to deduce, except quite empirically, from the present state of a cell what the succeeding one will be, that is, from being able to understand the succession of events as a necessary nexus which could be predicted. How a biophor comes to develop from itself the phenomena of life is quite unknown to us; we know neither the interaction of the ultimate material particles nor the forces which bring it about; we cannot tell what moves the hordes of different kinds of biophors to range themselves together in a particular order, what molecular displacements and variations arise from this, or what influence the external world has, and so forth. We see only the visible outcome of an endless number of invisible movements—growth, division, multiplication, reconstruction, and differentiation.
As long as we are so far from an understanding of life no theory of regeneration can be anything more than a 'portmanteau theory,' as Delage once expressed himself in relation to the whole theory of inheritance, a theory which is like a portmanteau in that one can only take out of it what has previously been put in. If we wish to explain the renewal of the aboral band of cilia in a Stentor, we first pack our trunk, in this case the nucleus of the Infusorian, with the determinants of the ciliated region, and then think of these as being liberated by the stimulus of wounding, and being brought to and arranged in the proper place by unknown forces to reconstruct the ciliary region in some unknown way. No one could be more clearly aware than I am that this is not an exhaustive causal explanation of the process itself. Nevertheless, it is not quite without value, inasmuch as it allows us at least to bring the facts together in rational order—in this case the dependence of the faculty of regeneration on the presence of nuclear substance—under a formula which we can use provisionally, that is, with which we can raise new questions. As soon as we ascend higher in the series of organisms the theory gains a greater value, for, while we leave altogether out of account any answer to the ultimate question, and thus renounce for the present the attempt to find out how the determinants set to work to call to life the parts which they control, we are brought face to face with other, in a sense, preliminary questions which we can solve, and the solution of which seems to me at least not entirely without value.
The first of these questions runs thus: Is the power of regeneration a fundamental, primary character of every living being in the sense that it is present everywhere in equal strength, independently of external conditions, and thus is an inevitable outcome of the primary characters of the living substance? Or is it, though primaeval in its beginnings, a phenomenon of adaptation, which depends on a special mechanism, and does not occur everywhere in equal extent and potency?
We have already become acquainted with some facts which must incline us to the latter view. The globular Alga-colonies of Volvox ([Fig. 63]) consist of two kinds of cells, of which only one kind, the reproductive cells, possess the power of reproducing the whole, the others, the flagellate, or, as we called them, somatic cells, being only able to produce their like, but never the whole.