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REGENERATION OF LOST PARTS
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REGENERATION OF LOST PARTS . A loss and renewal of living material, either continual or periodical, is a See also:familiar occurrence in the tissues of higher animals. The See also:surface of the human skin, the inner lining of the mouth and See also:respiratory See also:organs, the See also:blood corpuscles, the ends of the nails, and many other portions of tissues are continuously being destroyed and replaced. The See also:hair of many mammals, the feathers of birds, the epidermis of See also:reptiles, and the antlers of stags are See also:shed and replaced periodically. In these normal cases the regeneration depends on the existence of See also:special formative layers or See also:groups of cells, and must be regarded in each See also:case as a special See also:adaptation, with individual limitations and peculiarities, rather than as a See also:mere See also:exhibition of the fundamental See also:power of growth and See also:reproduction displayed by living substance. Many tissues, even in the highest animals, are capable of replacing an abnormal loss of substance. Thus in mammals, portions of See also:muscular See also:tissue, of epithelium, of See also:bone, and of See also:nerve, after accidental destruction or removal, may be renewed. The characteristic feature of such cases appears to be, in the higher animals at any See also:rate, that lost cells are replaced only from cells of the same morphological See also:order—epiblastic cells from the epiblast, mesoblastic from the mesoblast, and so forth. It is also becoming clear that, at least in the higher animals, regeneration is in intimate relation with the central See also:nervous See also:system. The See also:process is in See also:direct relation to the See also:general power of growth and reproduction possessed by See also:protoplasm, and is regarded by pathologists as the consequence of " removal of resistances to growth." It is much less See also:common in the tissues of higher See also:plants, in which the adult cells have usually lost the power of reproduction, and in which the regeneration of lost parts is replaced by a very extended capacity for budding. Still, more complicated reproductions of lost parts occur in many cases, and are more difficult to understand. In See also:Amphibia the entire epidermis, together with the slime-glands and the integumentary sense-organs, is regenerated by the epidermic cells in the vicinity of the defect. The whole See also:limb of a See also:Salamander or a See also:Triton will grow again and again after amputation. Similar renewal is either rarer or more difficult in the case of See also:Siren and See also:Proteus. In frogs regeneration of amputated limbs does not usually take See also:place, but instances have been recorded. Chelonians, crocodiles and See also:snakes are unable to regenerate lost parts' to any extent, while lizards and geckoes possess the capacity in a high degree. The capacity is absent almost completely in birds and mammals. In coelenterates, See also:worms, and tunicates the power is exhibited in a very varying extent. In See also:Hydra, Nais, and Lumbriculus, after transverse See also:section, each See also:part may See also:complete the whole See also:animal. In most worms the greater, and in particular the anterior part, will grow a new posterior part, but the separated posterior portion See also:dies. In Hydra, sagittal and See also:horizontal amputations result in the completion of the separated parts. In worms such operations result in See also:death, which no doubt may be a mere consequence of the more severe See also:wound. Extremely interesting instances of regeneration are what are called Heteromorphoses," where the removed part is replaced by a dissimilar structure. The tail of a See also:lizard, grown after amputation, differs in structure from the normal tail: the See also:spinal See also:cord is replaced by an See also:epithelial See also:tube which gives off no nerves; the vertebrae are replaced by an unsegmented cartilaginous tube; very frequently " super-regeneration " occurs, the amputated limb or tail being replaced by See also:double or multiple new structures. J. Loeb produced many heteromorphoses on See also:lower animals. He lopped off the See also:polyp See also:head and the pedal disc of a Tubularia, and supported the lopped See also:stem in an inverted position in the See also:sand; the See also:original pedal end, now See also:superior, gave rise to a new polyp head, while the See also:neck-end, on regeneration, formed a pedal disc. In Cerianthus, a See also:sea-See also:anemone, and in Cione, an ascidian, regeneration after his operations resulted in the formation of new mouth-openings in abnormal places, surrounded by elaborate structures characteristic of normal mouths. Other observers have recorded heteromorphoses in See also:Crustacea, where antennulae have been regenerated in place of eyes. It appears that, in the same See also:fashion as more simply organized animals display a capacity for reproduction of lost parts greater than that of higher animals, so embryos and embryonic structures generally have a higher power of renewal than that displayed by the corresponding adult organs or organisms. Moreover, experimental See also:work on the See also:young stages of organisms has revealed a very striking See also:series of phenomena, similar to the heteromorphoses in adult tissues, but more extended in range. H. Driesch, O. Hertwig and others, by separating the segmentation See also:spheres, by destroying some of them, by compressing young embryos by See also:glass plates, and by many other means, have caused cells to develop so as to give rise to structures which in normal development they would not have formed. It is clear that there are at least three kinds of factors involved in regeneration. There are: (1) Regenerations due to the presence of undifferentiated, or little differentiated, cells, which have retained the normal capacity of multiplication when conditions are favourable. (2) Regenerations due to the presence of special complicated rudiments, the stimulus to the development of which is the removal of the fully formed structure. (3) Regeneration involving the general capacity of protoplasm to See also:respond to changes in the surroundings by changes of growth. The most general view is to regard re-generations as special adaptations; and A. See also:Weismann, following in this See also:matter See also:Arnold See also:Lang, has See also:developed the See also:idea at considerable length, and has found a place for regenerations in his system of the germ-plasm (see See also:HEREDITY) by the conception of the existence of " See also:accessory determinants." Hertwig, on the other See also:hand, attaches See also:great importance to the facts of regeneration as See also:evidence for his view that every See also:cell of a See also:body contains a similar essential plasm. In E. Schwalbe's Morphologic der Minbildungen (1904), part i. See also:chap. v., an See also:attempt is made to See also:associate the facts of regeneration with those of See also:embryology and See also:pathology. Our knowledge of the facts, however, is not yet systematic enough to allow of important general conclusions. The power of regeneration appears to be in some cases a special adaptation, but more often simply an expression of the general power of protoplasm to grow and to reproduce its See also:kind. It has been suggested that regenerated parts always represent ancestral stages, but there is no conclusive evidence for this. view. (P. C. Additional information and CommentsThere are no comments yet for this article.
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