Regeneration in Animals

tail, grow, autotomy, nerve, legs, structure, cut, tion, leg and ganglion

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But there is yet another way of looking at these facts. What influence has function on morphological repair? In certain cases there is little doubt of the positive influence func tion may have on the quicker attainment of the normal form. When the tail of a newt is cut off by an oblique operation a bud will first grow at right angles to the cut surface, thus giving a crooked appearance. This abnormality will be adjusted quicker if the newt is allowed to swim about actively, than otherwise. The tentacles of digging forms of actinians are, after loss, not regen erated in their whole length if the animals are prevented from burrowing. In these cases it is clear that function modifies the muscles in the same way as it does during every exertion. On the other hand, the first stages of regenerating organs are often incapable of functioning. Antennae, claws and legs of crustaceans are, as regrowing buds, inclosed in a chitinous sack, thus being pre vented from moving or even receiving external stimulation. Nor does function apply, when the head of a flatworm has been re moved and new eyes develop in a tissue that has not before acted as a light-receiving organ.


Although these facts seem to prove conclusively that function cannot create form, but only strengthen the grow ing part, we have yet to consider the possibility that the nerves connecting a new growth with their centres pass stimuli of differ entiation as they do other commands to the periphery. It has been and is still held by some biologists that there are valid proofs of such a dependence of regenerating organs on a special nerve • or ganglion which normally innervates the structure which has to be restored. The stalk-eyed crustaceans have a ganglionic swell ing of the eye-nerve inside the stalk. If the stalk is removed or the ganglion extracted and the eye itself taken away, no regenera tion of the eye will make its appearance in adult specimens, al though these will go on moulting. In place of the eye a feeler like structure begins to grow, developing into a form most like that of the first normal antenna. This can be interpreted as a formative influence exerted by the antennal nerve on the bud in the cases where connection is made between the antennal ganglionic centre and the feeler-like structure. But this is very rare ; gener ally no such relation appears. Moreover, the nerve grows from the regenerating structure centripetally and the outgrowth could therefore only receive the stimulation of the antennal centre through nervous contact after completion of the feeler. In young stages eyes grow even after removal of the whole stalk, and here a formative stimulus could only be supposed to act in the same way as in older animals if we suppose the optic ganglion to regen erate first and then to transmit the determining influence distally. In short, a somewhat similar replacement of one organ by another in regeneration has been shown to depend on the removal of a particular joint including a sense organ: the antenna of the stick insects reappears as such if it is removed distally to the second joint. But if this, too, bearing the so-called Johnston's organ, has been taken away, the regrowth will resemble the first leg of the animal. This will happen even in individuals operated on im mediately after leaving the egg-shell. In other insects only late stages restore a foot-like structure in such cases, whereas larvae operated on early will restore the antenna itself after total extirpa tion. In mantids the foot-like structure can be identified as a leg of the first pair by the broad femur and toothed tibia as com pared with the slender, smooth appearance of the other two pairs. Besides the optic ganglion in crustaceans and Johnston's organ in insects, other nervous structures seem to be necessary for the growth of specialized parts; some forms of Nemertine worms do not regenerate if the brain has been removed, some flatworms do not grow new eyes before the cephalic ganglion has been restored. But in closely related species or even races there is no such restric tion to be observed. Many experiments have been conducted to ascertain the role played by the nervous system in the regrowth of the newt's limbs. It has now been conclusively proved that a special nerve-centre or nerve-path is not necessary, but that some sympathetic nerves must reach the cut surface before differentia tion of the bud begins. By deviating such a nerve the growth of a limb can be induced even in wounds made near the hind-leg but not including it. When the same nerve is made to innervate a cut in the tail it will induce an additional tail to grow. This seems

to prove that the nerve does not induce a special form which is, on the contrary, inherent in the different regions of the body. Could we not suppose that this same explanation covers the above-mentioned facts concerning invertebrate animals? Autotomy.—Whereas function cannot well have given rise to regeneration, a body of facts points towards its having induced special adaptations on the part of its owners. Some appendages are easily broken, for instance the lizard's tail. This apparently serves as a means of escape. As the reptiles, to which lizards be long, are believed to descend from amphibia-like ancestors which do not, as far as we can tell, include forms with easily-lost tails, although they are sure to have regenerated their legs as the living amphibians do, the power of easy breakage must have been ac quired during evolution. The rupture of the lizard's tail is effected at preformed planes in every single vertebra, not between verte brae. It is governed by nervous impulse inducing violent con traction of muscles which constrict and snap the vertebra. The regenerating tail does not replace the lost part of this or any other vertebra, a continuous cartilaginous rod running through it. It may be added that regeneration can also take place if a part of this new tail without preformed planes of breakage is re moved or the original tail is cut off between two vertebrae, another proof of the priority of the regenerative faculty and its inde pendence of easy loss. Similar adaptations are also presented by the legs of crayfish and some insects. Generally a furrow runs • round the second joint and furnishes the same ability of snapping as the preformed plane of the lizard's tail. In both cases, on seizure near the distal end of the appendage being felt, the ani mal struggles to get away, and the pull together with the reflex action suffices to get rid of the encumbrance. The constriction of the wound serves at the same time to prevent bleeding and is therefore of use for retaining the resources of restoration; more over, regeneration can begin earlier and proceed quicker and more regularly than from irregular profusely bleeding wounds. But, on the other hand, the mechanism of this autotomy (self-amputa tion) may close the wound so thoroughly by telescoping one joint into the other that no regenerating bud can pierce its way. Thus, in grasshoppers and crickets, the hind legs are easily lost but do not regenerate after autotomy, whereas the two front pair of legs regenerate though incapable of autotomy. But if the whole hind leg, including the first two joints, be excised, thus obviating the telescopic action, a miniature appendage makes its appearance. This proves that the regenerative faculty of the hind-legs is not really lost. In the stick insect all three pairs of legs are adapted for autotomy and regenerate not only from the preformed snap ping plane, but also distal or proximal to it. It seems possible to prevent regeneration, at least in the middle leg, by removing it through a plain vertical cut, which is quickly covered over by chitinous plates smothering the underlying tissue that could other wise grow out into the bud of the leg. Thus regeneration is al ways primarily present, secondary adaptations either modifying or preventing it from taking place. Autotomy serves merely its first purpose of allowing the animal to escape its foe, but is unable to restore the lost member. The adaptation might thus appear to be not worth while, as the mutilated insect, permanently encumbered in its movements, would be at a disadvantage and unable to propagate the species. Some examples show that dep osition of eggs may occur shortly after the mutilation, for in stance when fully-developed grasshoppers have lost a hind leg. Regeneration cannot set in, as no insect regenerates members after its metamorphosis. For the purpose of propagation all that is necessary is escape from the instant peril. In some worms autotomy directly serves the purpose of disseminating the genital products. At certain times of the year millions of pieces from the Palolo worms rise to the surface of the southern Pacific and re produce their kind. In certain squids, a specially adapted arm of the male, the so-called hectocotylus, is detached and carries the male products to their destination. Autotomy may also lead to asexual reproduction; thus in actinians, flatworms, starfish and annelid-worms pieces give rise to new animals by regeneration. Often rudiments of structures to be renewed in the posterior piece may be seen before severance from the anterior as in the fission of infusorians.

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