REGENERATION OF LOST PARTS  . A loss and renewal of living material, either continual or periodical, is a

familiar occurrence in the tissues of higher animals . The surface of the human skin, the inner lining of the mouth and respiratory organs, the blood corpuscles, the ends of the nails, and many other portions of tissues are continuously being destroyed and replaced . The hair of many mammals, the feathers of birds, the epidermis of reptiles, and the antlers of stags are shed and replaced periodically . In these normal cases the regeneration depends on the existence of special formative layers or groups of cells, and must be regarded in each case as a special adaptation, with individual limitations and peculiarities, rather than as a mere exhibition of the fundamental power of growth and 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 muscular tissue, of epithelium, of bone, and of nerve, after accidental destruction or removal, may be renewed . The characteristic feature of such cases appears to be, in the higher animals at any rate, that lost cells are replaced only from cells of the same morphological 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 nervous system . The process is in direct relation to the general power of growth and reproduction possessed by protoplasm, and is regarded by pathologists as the consequence of " removal of resistances to growth." It is much less common in the tissues of higher 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 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

limb of a Salamander or a Triton will grow again and again after amputation . Similar renewal is either rarer or more difficult in the case of Siren and Proteus . In frogs regeneration of amputated limbs does not usually take place, but instances have been recorded . Chelonians, crocodiles and 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, worms, and tunicates the power is exhibited in a very varying extent . In Hydra, Nais, and Lumbriculus, after transverse section, each part may complete the whole animal . In most worms the greater, and in particular the anterior part, will grow a new posterior part, but the separated posterior portion dies . In Hydra, sagittal and horizontal amputations result in the completion of the separated parts . In worms such operations result in death, which no doubt may be a mere consequence of the more severe 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 lizard, grown after amputation, differs in structure from the normal tail: the spinal cord is replaced by an epithelial 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 double or multiple new structures .

J . Loeb produced many heteromorphoses on

lower animals . He lopped off the polyp head and the pedal disc of a Tubularia, and supported the lopped stem in an inverted position in the sand; the original pedal end, now superior, gave rise to a new polyp head, while the neck-end, on regeneration, formed a pedal disc . In Cerianthus, a sea-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 Crustacea, where antennulae have been regenerated in place of eyes . It appears that, in the same 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 work on the young stages of organisms has revealed a very striking 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 spheres, by destroying some of them, by compressing young embryos by 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

respond to changes in the surroundings by changes of growth . The most general view is to regard re-generations as special adaptations; and A . Weismann, following in this matter Arnold Lang, has developed the idea at considerable length, and has found a place for regenerations in his system of the germ-plasm (see HEREDITY) by the conception of the existence of " accessory determinants." Hertwig, on the other hand, attaches great importance to the facts of regeneration as evidence for his view that every cell of a body contains a similar essential plasm . In E . Schwalbe's Morphologic der Minbildungen (1904), part i. chap. v., an attempt is made to associate the facts of regeneration with those of embryology and 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 kind . It has been suggested that regenerated parts always represent ancestral stages, but there is no conclusive evidence for this. view . (P . C .