It is clear, then, that the faculty of regeneration is not a primary character of living beings occurring uniformly in all species of equally high organization and in all parts of an animal in the same degree; it is a power which occurs in animals of equal complexity in as varying degrees as in their parts, and which is manifestly regulated by adaptation. Between parts with the faculty of regeneration and parts without it there must be an essential difference; there must be present in the former something that is wanting in the latter, and, according to our theory, this is the equipment with regeneration-determinants, that is, with the determinants of the parts which are to be reconstructed.
If this be really so it should be capable of proof, at least in so far that we should be able to establish that the power of completing or re-forming a damaged or lost part is a limited one, localized in certain parts and cell-layers. This can be actually proved, as may be seen from numerous cases in which the faculty of regeneration is associated with autotomy, that is, with the power of breaking off or dropping off a part of the body. Even in worms we find this power, as we mentioned before in speaking of the high regenerative capacity of Lumbriculus. This worm reproduces in summer by what is called 'schizogony,' that is, by breaking into two, three, or more pieces, and it does not seem to require a very strong stimulus, such as pressure of the end of the worm by the jaws of an insect larva, to start this rupture; it often follows from quite insignificant friction on the ground. Certainly the power of regeneration is so great in this animal that it is out of the question to talk of localizing the primary constituents of regeneration; almost every broken surface is capable of regeneration.
But this localization is well illustrated in Insects and Crustaceans, which possess the power of self-amputation in their appendages, especially in their legs. As far back as 1826 MacCullock observed this remarkable power in crabs, and described the mechanism on which it depends. When the leg is irritated, for instance when it is pinched at the tip and held fast, it breaks off at a particular place. This line of breakage lies in the middle of the short second joint (Fig. 98, A and B, s), just between the insertions of the muscles (me, mf, m) which extend from this line towards the extremity of the limb and in the opposite direction towards the body-wall. Between these muscle-attachments the external skeleton is thin and brittle, and forms a suture, s, which breaks through when the animal contracts the muscles of the leg convulsively, and thus presses the lower protuberance (a) against a projection (b) of the first upper joint. Crabs require to make a very considerable muscular exertion before they can throw off the limb, and therefore they can only do it when they are in full vigour.
Fig. 98. The leg of a Crab,
adapted for self-mutilation or autotomy.
A, the first three joints of the limb, I, II, III. s, the
suture, that is, a thin area on
the second joint which is predisposed
to breakage. mf, flexor
muscle, me, extensor muscle, both
inserted at the suture. B, the
entire leg with its six joints and
with the suture (s). Slightly enlarged.
After MacCullock.
We have here a quite definite structural adaptation of the parts to a danger which often recurs—that of falling entirely into the power of an enemy which has seized the leg. By a sudden violent throwing-off of the leg the crab escapes from this danger. Quite similar adaptations are found among certain insects, such as the walking-stick insects or Phasmids, in which the mechanism is much the same, and lies at an almost exactly corresponding place, namely, at the line where the second and third joints of the leg, the 'trochanter' and the 'femur' meet. In this case the advantage of the arrangement is not merely that the animals are thus enabled to escape from enemies; it is useful in another connexion, for a knowledge of which we have to thank Bordage. This naturalist observed that the Phasmids not infrequently perished at one of their numerous moultings, by remaining partially fixed in the discarded husk. Of 100 Phasmids nine died in this way, twenty-two got free with the loss of one or more legs, and only sixty-nine survived the moult without any loss at all.
That the moulting or ecdysis of insects is often hazardous may be observed in our own country, and it is familiar to every one who has reared caterpillars. These, too, often fail to get clear of their 'cast' cuticle, and they perish unless artificial aid is given to them. I have never observed any autotomy in them, but in the Phasmids it seems to be a much-used 'device' and is therefore of great importance in the persistence of the species.
Limbs which are thus thrown off by autotomy regenerate again from the place at which they broke off, that is from the 'suture.' It had been noticed even by the earlier observers (e.g. Goodsir) that there was a jelly-like mass of cells within the joint, and that the development of the new limb started from this. It might be supposed that the regeneration-primordium is present in the rest of the leg also, but that is not the case, for the animal responds to the tearing off of one joint or of a smaller number than to the suture, not by regenerating the torn part directly, but by amputating the whole of the leg up to the suture, and then from this the regeneration of the whole leg takes place. In the Phasmids the case is similar, but with the difference that regeneration is possible from three places, from the tarsal joints, from the lower third of the tibia, and finally, from the suture between the femur and the trochanter. There is thus a regeneration-primordium (Anlage) at the beginning of the tarsal joints, another in the tibia, and a third in the 'suture' and the first must be equipped, as we should express it, with the determinants of the five tarsal joints, the second with those for the lower end of the tibia as well, and the third with all the determinants of the whole leg, from the 'suture' downwards.
In any case, regeneration is here associated with definite localized pieces of tissue, and is not a general character of all the cells of the leg, and, as it obviously runs parallel at the same time with another adaptation—that of autotomy—there can be no doubt that it too is dominated by the principle of selection, and that it can not only be increased, but that it can be concentrated at particular places and removed from others. But this is only possible if it be bound up with material particles which may be present in or absent from a tissue, and which are therefore a supplement to the ordinary essential constituents of the living cells, although they do not themselves belong to the essential organization.