New investigations which have been carried out by Dr. Otto Hübner in my Institute have placed these facts beyond doubt. We may conclude that, in this case, a disintegration of the germ-plasm has taken place during ontogeny, by means of differential cell-division, so that only the reproductive cells receive the complete germ-plasm, while the somatic cells receive only the determinants necessary to their own specific differentiation, the somatic determinants.
In this case regeneration and reproduction coincide; there is no regeneration except the origin of a new individual from a reproductive cell.
Fig. 35 B (repeated). Hydra viridis,
the Green Freshwater Polyp.
Section through the body-wall,
somewhere in the direction of ov
in Fig. 35 A. Eiz, the ovum lying in
the ectoderm (ect), and including
zoochlorellæ (schl) which have immigrated
from the endoderm (ent)
through the supporting lamella
(st). After Hamann.
Let us now ascend to the lowest of the Metazoa, for instance, the freshwater polyp, Hydra ([Fig. 35 A]), and we find a high degree of regenerative capacity in the restricted sense, for, in addition to the power of producing germ-cells, that is, cells which, when two combine in amphimixis, give rise again to a new animal, almost any part of the polyp can regrow a whole animal. Not only has Hydra been cut in from two to twenty different pieces, but it has even been chopped up into innumerable fragments, and yet each of these, under favourable circumstances, was able to grow again into a complete animal. Nevertheless, we are not justified in concluding that every cell possesses the power of reproducing the whole. If, with the help of a bristle, we turn one of these polyps outside in like the finger of a glove, and then prevent it turning right again by sticking the bristle transversely through it, it does not live, but soon dies, obviously because the cells of the two layers of the body, ectoderm and endoderm, cannot mutually replace each other, and cannot mutually produce each other. The inner layer, now turned outwards, cannot resist the influence of the water, and the outer layer, now turned inwards, cannot effect digestion; in short, one cannot be transformed into the other, and we must therefore conclude that both are specialized, that they no longer contain the complete germ-plasm, but only the specific determinants of ectoderm and endoderm respectively.
The animal's high regenerative capacity must therefore depend on the fact that certain cells of the ectoderm are equipped with the complete determinant-complex of the ectoderm, in the form of an inactive accessory idioplasm, which is excited to regenerative activity by the stimulus of wounding, and that, in the same way, the cells of the endoderm are equipped with the whole determinant-complex of the endoderm. It need not be decided whether all or only many of the cells, perhaps the younger ones, are thus adapted for regeneration; in any case a great many of them must be distributed throughout the whole body, with perhaps the exception of the tentacles, which are by themselves unable to reproduce the whole animal. When the animal is mutilated, the cells of both layers, equipped with their respective determinant-aggregates, co-operate in reproducing the whole from a part.
It is true that even with these assumptions we only reach the threshold of a real explanation. For, given that all the determinants of the species must be present in a fragment, we are not in a position to show how these set about reconstructing the animal in its integrity, and the most that we can say is, that it must depend on the specific kind of stimulus to which each of the cells is exposed through its direct and more remote environment, which determinants are to be first liberated, and therefore which parts are to be reconstructed.
That there are at work regulative forces, such as we were already compelled to assume in regard to the division and regeneration of unicellular organisms, as to the nature of which we cannot yet make any definite statement, but which we may call 'polarities,' or, as I prefer to say, 'affinities,' is shown by countless experiments which have been made, particularly with the freshwater polyp. Thus Rand cut off the anterior end of the polyp with its circle of tentacles, and the excised disk of living substance lengthened in a transverse direction, so that half the tentacles came to lie to the right, the other half to the left, while the body developed between these two groups, so that they became further and further separated from each other, till finally the original transverse axis of the animal became the longitudinal axis. One group of tentacles survived and surrounded the new mouth, while the other at the opposite aboral pole, the new foot, died off. This total change of structure in the polyp, as to the arrangement of its main parts, points to unknown forces, which cannot depend on the determinants as such, but on the vital characters of the living parts, and on the interactions of these with one another.
Fig. 96. A Planarian cut transversely into nine pieces. The regeneration of seven of these into entire animals is shown. After Morgan.