Fig. 100. Regeneration of Planarians. A, an animal divided into three parts by two oblique cuts. B, the fragments(a, b, c) in process of regeneration. C, an animal with various oblique incisions in the margin of the body, which have induced the new formation of heads (k), of tails (s), and pharynx (ph). A and B after Morgan; C after Walter Voigt.
Walter Voigt has shown, by a series of most interesting experiments, that it is possible not only to cause the development of a new head in Planarians by cutting them, in which case a tail may grow from the anterior portion and a head from the posterior portion, but it is also possible in an intact animal, that is, one with both head and tail, to cause the production of a second head, or a second tail, or both at once, at any part of the body margin at will, according to the direction of the cut. If the margin of the body be cut obliquely forwards (Fig. 100, A) a supernumerary tail arises (C, s), if it be cut obliquely backwards a supernumerary head arises (C, k), and in this way several heads and several tails may be produced in the same animal. It is obvious, then, that the interaction, in the first place, of the cells of the cut surface, but probably also of the deeper-lying cells, decides which determinants are to come into action, those of the head or those of the tail, but both must be present at every part of the cut. How far below the cut surface the cells take part in this determination we cannot make out, but that it cannot be due to the co-operation of all parts is clear in this case at least, since the animal still possesses its original head and tail. The extra heads and tails thus produced prove, at any rate, that there can be no question here of the expression of an adaptive principle, a spiritus rector, or a vital force, which always creates what is good, but that it is rather a purely mechanical process, which takes its course quite independently of what is useful or disadvantageous, and that it must take this course according to the given regeneration-mechanism and the stimulus supplied in the special case. It cannot be supposed that these supernumerary heads and tails are purposeful, but who would expect an adaptive reaction from the animal in a case like this, since cuts of the kind which we make artificially, and must keep open artificially if the deformities are to develop, hardly occur in nature, and, if they did occur, would very quickly close up again? Adaptations can only develop in response to conditions which occur and recur in a majority of cases, and when they have a useful, that is, species-preserving result. The adaptiveness of the organism is blind, it does not see the individual case, it only takes into account the cases in the mass, and acts as it must after the mechanism has once been evolved. The case is the same as that of 'aberrant' or mistaken instincts, whose origin by means of selection is the more clearly proved, since we must recognize such an instinct as a pure mechanism and not as the outcome of purposeful forces.
In the regeneration of Planarians we must think of the regeneration-idioplasm as containing the full complex of the collective determinants of the three germinal layers, and possibly we must add to this cells with the complete germ-plasm for giving rise to the reproductive cells. But when the amputated tail of the newt is regenerated, or its leg, or the arm of a starfish, or the bill of a bird, we have no ground for assuming that the cells, from which regeneration starts, contain the whole germ-plasm, since the determinants of the replaceable parts suffice to explain the facts. We must even dispute the possibility of the presence of the whole germ-plasm in this case, because the faculty of regeneration of the relevant cells is really no longer a general one, but is limited to the reproduction of a particular part. This is seen in the fact that, in the starfish, whose high regenerative capacity is well known, the central disk of the body may indeed give rise to new arms[2]; but an excised arm, to which no part of the disk adheres, is in most starfishes unable to give rise to the body. Thus the arm does not contain in its cells the determinants of the disk, but the latter contains those of the arm. We are not surprised that the amputated tail of the salamander does not reproduce the whole animal, but this can only be because the impelling forces to the regeneration of the whole animal are wanting, that is, that the cut surface only contains the determinants of the tail and not the complete germ-plasm. It might be objected here that the tail-piece is too small to give rise to the whole body, but in Planaria it is only very diminutive heads and tails which grow from the artificial incisions, and the same is true of starfishes when only a single arm and a small piece of the disk have been left. Notwithstanding the small amount of living substance at their disposal, and although they are at first unable to take nourishment, they send out very small new arms (Fig. 101), close up the wounded surface, and, after reconstruction of the mouth and stomach, begin to feed anew. The new arms may then grow to the normal size.
[2] I see now that there are contradictory statements in regard to this case. Possibly these depend on the different behaviour of different species, and this on the varying frequency of mutilation. Starfishes which live on the shore between the rocks, for instance on the movable stones of a breakwater, are very frequently mutilated; in some places it is rare to find a specimen without traces of former wounds. H. D. King counted among 1,914 specimens of Asterias vulgaris 206 in the act of regenerating a part, that is, 10.76 per cent. In the case of the starfishes from deep water this cause of injury does not of course exist.
Fig. 101. A starfish arm,
growing four new arms;
the so-called 'comet-form.'
After Haeckel.
We must therefore assume that, in many cases, the regeneration-primordium consists of cells which only contain a definite complex of determinants in the form of latent regeneration-idioplasm, as, for instance, certain cells of the tail of Triton contain the determinants of the tail, certain cells of its leg the determinants of the leg, and so on. In many cases we can speak even more precisely, and determine from which cells the nerve-centres, from which the muscles, and from which the missing section of the food-canal will be formed, as was recently shown by Franz von Wagner in regard to the worm Lumbriculus, whose regenerative capacity is so extraordinarily high. We must then attribute to each of the relevant cells an equipment of regeneration-idioplasm, which includes only the relevant complex of determinants.
I need not here go further into detail, but I should still like to show that, in reality, as I assumed in regard to the regenerative capacity of a part, the root of the regeneration-idioplasm lies in the germ-plasm, that it is present there as an independent determinant-group, and, like every other bodily rudiment (Anlage), must be handed on from generation to generation. This assumption is necessary, as has been already indicated, on the ground that the faculty of regeneration is hereditary, and hereditarily variable, on the same ground, therefore, as that on which the whole determinant theory is based. The regeneration-determinants must be contained as such in the germ-plasm, otherwise a twofold phyletic development could not have occurred, as it actually has, in many parts. The tail of the lizard is adapted for autotomy; it breaks off when it is held by the tip, and this depends on a special adaptation of the vertebræ, which are very brittle in a definite plane from the seventh onwards. This is thus a very effective adaptation to persecution by enemies. The tail which has been seized remains with the pursuer, but the lizard itself escapes, and the tail grows again. But this regeneration does not take place in the same way as in the embryo; no new vertebræ are formed, but only a 'cartilaginous-tube,' a new structure, a substitute for the vertebral column; the spinal cord with its nerves is not regenerated either, and the arrangement of the scales is somewhat different.