The Encyclopedia Americana (1920)/Regeneration (zoology)
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REGENERATION, in zoology, signifies the reproduction, or natural restoration, of parts of the body, whether external or internal, lost by injury. This power of renewal belongs in some degree to all living creatures, and is one of the distinguishing features of organic, as distinct from inorganic, nature. A broken crystal may rebuild itself when it is allowed to remain for a suitable time in a saturated solution of the mineral of which it consists; but this renewal is accomplished wholly by external accretion of new material, whereas an animal supplies a lost part wholly by the ordinary process of food-assimilation and growth.
This power is most manifest, as might be predicted, in the animals of the simplest organization, and in those most exposed to serious mutilation, and decreases in a varying scale as organisms become more and more complex, until in the higher vertebrates (except lizards) it is limited to the healing of wounds. (The success of what surgeons call grafting, that is, the organic attachment of new flesh to old, as when detached skin is placed over a flayed surface, might be classified here, but it is an artificial utilization of the regenerative principle).
Among such lowly animals as protozoans, cœlenterates, echinoderms and worms, the power of regeneration is very great and of much importance, although it appears with much inequality and with some strange irregularities; among crustaceans it is strong in some groups and not in others; and among the vertebrates is confined mainly to the amphibians and the lizards.
As long ago as the middle of the 18th century it was learned that if a hydra be sliced into thin cross-sections from each ring would sprout a fringe of tentacles, and when cut lengthwise into strips each strip would form for itself a complete hydra. The same thing happens when protozoans, and even some worms, are dissected. “If,” says Morgan, “a fresh-water worm (Lumbriculus) is cut into pieces, each piece makes a new head at its anterior end, and a new tail at the posterior end; in this way as many new worms are produced as there are pieces.” Not all worms are so energetic. Thus in the common earthworm the cutting must be done carefully. “If from one to five of the anterior segments be cut off,” according to Packard, “the same number comes back; if more are cut off the process of regeneration begins only after a longer interval, and only four or five segments come back as a rule; if the cut be behind the middle . . . fewer worms succeed in regenerating at all.” In the case of another annelid (Allolobophora) allied to the earthworm, when cut in two in the middle the posterior piece "regenerates at its anterior cut end not a head but a tail." This is one of the many curious examples of inconsistency or irregularity in this function. In the hydroids the presence or absence of light has much to do with the form regeneration will take, and elsewhere other external conditions are influential.
Among the echinoderms regeneration is most conspicuous is the starfishes. Before this ability was generally known the oystermen of Long Island Sound, whose planted oyster-beds were infested with destructive starfishes, used to catch great numbers of them by dragging “tangles”; piling them on the deck of the boat they would, as an easy means of dispatching them quickly, saw through the heaps by pulling a coarse cord and then throw the severed halves overboard. The result was that each half of the starfish (unless its centre was utterly ruined) put forth new arms, and thus the number of pests was doubled instead of decreased. Now the starfishes caught are taken ashore and turned into fertilizer.
Crabs and other crustaceans will develop new limbs to replace lost ones. “If,” says Calman (‘Life of Crustacea,’ 1911), “a lobster be caught by one of its claws or by a leg, it very readily parts with the limb in its struggles to escape”; and he explains that the connection at the joint of the second and third segments is slight, and that there is only a small hole through which blood vessels and nerves pass to the distal segments, and this is quickly plugged by a blood-clot. “Beneath the scar which forms on the stump of a separated limb a sort of bud grows, and gradually assumes the form of the lost segments . . . and ultimately provides in normal cases a new member, similar in every detail to that which has been lost.”
Amphibians, especially salamanders and the young frogs, suffer greatly from mutilation, not only by enemies who catch them by the tail or a limb, which may be lost in the effort to escape (in some cases by voluntary sloughing off of tail or gills), but by the biting of their own fellows, especially when many larvæ are associated. Salamanders are particularly subject to loss or mutilation of their long tails. These, and lost hands, are restored in the course of three or four months; but in place of the bones that formerly existed in the part a similar cartilaginous framework only is supplied. Fully grown frogs are slower than the young and often the restoration is incomplete.
The same is true of lizards, whose tails are frequently torn or cast off and regenerated. The new structure is, however, as Gadow says, “a sham tail, since neither new centra nor arches [of the vertebræ] but only a non-segmented rod or tube of fibro-cartilage is produced.” Gadow (‘Amphibia,’ 1901) continues: “The regenerated tail is, however, provided with new muscles, and with skin, but the scales often differ considerably from those of the normal organ. Boulenger has found that the new r aberrant scaling is in some cases a reversion to an ancestral form. This, for instance, is the case in Pseudopus, and in the tejoid genus Gymnophthalmus; to a certain extent also in geckos and skinks. On the other hand Lacertidæ, Gherrosauridæ and also Anguidæ, reproduce a caudal scaling true to their type. Injured or broken-off tails are often reproduced double or even trifid; sometimes an additional little tail grows out from an injured spot, anywhere on the side of an injured or mended tail.”
Turtles will mend injured plates of their shell, or in some cases are able to reproduce a plate wholly lost.
The physiological process or method by which regeneration is effected through the cells at the surface of the injury or amputation is still a matter of theory. The latest statement of the power of regeneration manifested by the cells concerned is that by Henry Fairfield Osborne (‘Origin of Life and Evolution,’ 1917), who believes that such “regeneration is attributable to the potentiality of the hereditary chromatin which still resides in the cells of the amputated surfaces.” It has been argued by some of the stricter followers of Darwin that this power of regeneration is an acquirement through natural selection, but Morgan shows conclusively that this view is untenable. He speaks of the healing of wounds, which occurs in all animals, and asserts that its higher manifestation in the restoration of lost limbs is adaptive in character. “The immense usefulness of this power,” Morgan reminds us, “is obvious when it is remembered how exposed most animals are to injuries. By repairing the injury the animal can better carry on its normal functions. Moreover the presence of the wound would give injurious bacteria a ready means of entering the body. In fact, an intact skin is one of the best preventives to the entrance of bacteria.” Consult Morgan, T. H., ‘Regeneration’ (New York 1901).