REGENERATION (Lat. regeneratio, from regencrure, to generate anew, from back again, anew + generare, to beget, from genus, family). Replacement of lost parts. renewal of organs, or completion of an organism from a part. In 1744 the Swiss naturalist Trembley found that on cutting hydras in two, or slicing them into thin rings, from each ring grew out a crown of tentacles; and in splitting them into longitudinal strips each portion became a well shaped hydra. Finally he turned one inside out and in a few days the evacrinated hydra swallowed pieces of meat, though former stomach-lining had now become its skin. Bonnet found that from the same region of a worm, like the earth worm, a head or tail may arise according to whether that region happens to lie at the anterior or posterior end of the cut surface. Thus if a worm (Lumbriculus) he cut into two pieces, a new tail will develop from the posterior end of the anterior piece. and a new head from the front end of the posterior piece. In another species of fresh-water annelid Bonnet found that a new tail developed at the anterior end of the posterior piece, and not a head. As the result of recent experiments on the earthworm it is ascertained that if from one to five of the anterior segments be cut off, the same number come 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, time time before regeneration begins is still longer, and fewer worms succeed in regenerating at all. Each end of the body can regenerate in its own direction only.
The effect produced by external factors in experiments on regeneration are noteworthy. A hydroid (Eudendrium) failed to develop new heads when kept in the dark, but when placed in the light the new heads quickly appeared, show ing that light acts as a stimulus; and instead of a head a root will develop at the distal end of a piece if that end be brought into contact with some fixed object, and conversely a new head will appear at either the nearest or farthest end if the end be freely surrounded with water: in this case the external agent is the stimulus which determines the differentiation of the part; and experiments have produced some extremely curious manifestations of how this stimulus acts.
For example, a piece cut from the stern of Anten nularia and suspended vertically in the water will develop a new stem at the upper end, and roots at the lower end, regardless of the normal position. Here gravity alone determines that the upper end shall grow into a stern and the lower end into a new root. In the one case, where the upper end corresponds to the proximal end of the original hydroid, the new part (a stem) replaces the lost root-end, and this change Loeb calls `beteromorphosis! In the other case, says Morgan, where the upper end corresponds to the distal end of the original hydroid, the new part (a stein) replaces the lost part of the stem. This is what is usually meant by 'regeneration.' It is of some theoretical interest to know whether the old cells directly form the new tissue or whether reserve cells are present that bring about the result. From experiments thus far made Morgan thinks that this may vary with different forms. In Planaria, a flatworm, a new head forms on the anterior end of the posterior half of the worm, but the entire anterior half is never replaced by new tissue. In this same plana rian Randolph has discovered a most important relation existing between the old and new parts. If a planarian be cut in two longitudinally in the median plane, the right half regenerates a new left half of the same size as the part removed, and the left half also develops a new right half of corresponding size. lf, however, the worm be cut longitudinally into a larger and smaller strip, the former replaces as much a, was contained in the smaller part removed, but the smaller part does not develop the lost larger part, but forms only as much new tissue at its cut side as is about equivalent to its own breadth.