Regeneration

Although a few cases of regeneration were spoken of by Aris- totle and by Phny, the subject first attracted general attention through the remarkable observations and experiments of the Abbe Trembley.His interest was drawn to certain fresh-water polyps hydras that were new to him, and in order to find out if the organisms were plants or animals he tried the effect of cutting them into piece" for , was generally known that pieces of a plant made a new'pTant' but if a n animal cut mt0 pieceS; the p .eces Tremb P , that the polyp, .fcut m two, produced two polyps.Logically he should have concluded that the new form was T plant ^bu from other observatmns, as to its method of feeding and of movement Trembey concluded that the polyp was an animal, and that the animakl * MW °rganism fr°m a P art belong to ammals as well as to plants.» I felt," he says, » strongly that nafure toe .vast, and too little known, for us to decide without temerity REGEXERA TIOX Trembley found that when a hydra is cut in two, the time required for the development of the new individuals is less during warm than during cold weather.He also found that if a hydra is cut into three or four parts, each part produces a new individual.If these new hydras are fed until they grow to full size, and are then again cut into pieces, each piece will produce a new polyp.The new animals were kept in some cases for two years, and behaved in all respects as do ordinary polyps.Trembley also found that if the anterior, or head-end, with its tentacles, is cut off, it also will make a new animal.If a hydra is cut lengthwise into two parts, the edges roll in and meet, and in an hour, or less, the characteristic form may be again assumed.New arms may appear later on the new individual.If a hydra is split lengthwise into four pieces, each piece will also produce a new polyp.If the head-end only of a hydra is split in two, each half becomes )rms, one of which, at least, seems to have been the annelid lum- iculus.His first experiments (1741) showed that when the worm cut in two pieces, a new tail develops at the posterior end of the terior piece, and a new head at the anterior end of the posterior 1. -A-B.After Trembley.C-G'.After Bonnet. A. Seven-headed hydra made by splitting head-ends lengthwise.B. Illustrating the method of turning hydra inside out by means of a bristle: i, foot being pushed through mouth ; 2, completion of process.C. Middle piece of an earthworm (cut into three pieces) with new head and tail.D. Anterior part of an earthworm regenerating a new " delicate " tail.E. Posterior third of a worm (lumbriculus) that regenerated two heads.F. Middle piece of a worm (another species) cut into three pieces.It made a tail at each end.F. Anterior, enlarged end (taii) of last.G. Small piece of a worm.G' .Regeneration of head and tail of same.:e.He found that if a worm is cut into three, four, eight, ten, iven fourteen pieces, each piece produces a new worm ; a new d appearing on the anterior end of each piece, and a new tail on posterior end (Fig. i,G,G').The growth of the new head is ted in all cases to the formation of a few segments, but the new REGENERATION tail continues to grow longer, new segments being intercalated just in front of the end-piece that contains the anal opening.In summer the regeneration of a new part takes place in two to three days ; in winter in ten to twelve days, this difference not being due to the time of year, but to the temperature.Bonnet found that if a newly regenerated head is cut off, a new one regenerates, and if the second one is removed, a third, new one develops, and in one case this oc- curred eight times : the ninth time only a bud-like outgrowth was formed.In other cases a new head was produced a few more times, but never more than twelve.He thought that the capacity of a part to regenerate is in proportion to the number of times that the animal is liable to be injured under natural conditions.Bonnet found that short pieces from the anterior or posterior end of the body failed to regenerate, and usually died in a few days.Occasionally two new heads appeared at the anterior end of a piece (Fig. i, E\ and sometimes two tails at the posterior end.Another kind of fresh-water worm 1 was found that gave a very remarkable result.If it was cut in two pieces, the posterior piece produced at its anterior end, not a new head, but a new tail.Thus there is formed a worm with two tails turned in opposite directions, as shown in Fig. i,F,F f .Spallanzani made many experiments on a number of different animals, but unfortunately the complete account of his work was never published, and we have only the abstract given in his Prodromo (1768).He made a large number of experiments with earthworms of several kinds, and found that a worm cut in two pieces may pro- duce two new worms ; or, at least, that the anterior piece produces a new tail, which increases in length and may ultimately represent the posterior part of the body ; the posterior piece, however, produces only a short head at its anterior end, but never makes good the rest of the part that was lost.A short piece of the anterior end fails to regenerate ; but in one species of earthworm, that differs from all the others in this respect, a short anterior piece or head can make a new tail at its posterior end. 2 Spallanzani also found that if much of the anterior end is cut off, the development of a new head by the posterior piece is delayed, and, in some species, does not take place at all.If a new head is cut off, another is regenerated, and this occurred, in one case, five times.If, after a new head has developed, a por- tion only is cut off, the part removed is replaced, and if a portion of this new part is cut off it is also regenerated.If a worm is split 1 An annelid of unknown species.2 This statement of Spallanzani's I interpreted incorrectly ('99), thinking that he obtained a two-tailed form as had Bonnet.RE GENERA TION collar, may be regenerated.The head of the slug, it was found, regenerates with more difficulty than does that of the snail.These justly celebrated experiments of Trembley, Reaumur, Bonnet, and Spallanzani furnished the basis of all later work.Many new facts, it is true, have been discovered, and in many cases we have penetrated further into the conditions that influence the regeneration, but many of the important facts in regard to regeneration were made known by REGENERATIONIn the earthworm also we find some interesting facts connected with the regeneration of the terminal pieces.If one, two, three, four, or five segments are cut from the anterior end, they will die without regenerating.Pieces that contain more segments, six to ten, for example, may remain alive for a month or longer, but do not regenerate (Fig. 3,A,B).That this lack of power to regenerate at the posterior end is not due to the smallness of the piece can be shown by removing from a piece of five segments one or two of its anterior segments.These will be promptly regenerated.Another experiment Fig. 5. -Hydra viridis.A. Normal hydra.Lines indicate where piece was cut out.B, 1-4.Changes in a piece of A, as seen from the side.C, 1-4.Same as see'n from the end.D, E, F. Later stages of same piece, drawn to same scale.has shown, however, that if these small pieces can be kept alive for a long time, and also supplied with nourishment, regeneration will take place at the posterior end.If, for instance, a small piece of eight or ten segments has its anterior three or four segments cut off, and is grafted by its anterior end to the anterior end of another worm, as shown in Fig. 3, F, the piece will begin, after several months, to re- generate at its exposed posterior end, but in the one instance in which this experiment has been successfully carried out, a new head, and not a tail, appeared on the exposed free end.The result is not due to the grafting, or to the anterior position of the posterior end, but to REGENERATION as the new part grows larger (Fig. 4,E 2 , E z).In a land planarian, Bipalium kewense, a piece is transformed into a new worm, as shown in Fig. 6, A, B. In this case the old pigment stripes of the piece are carried directly over into the new worm, the piece elongating during the transformation.A similar change takes place in pieces of unicellular animals, as best shown by cutting off pieces of stentor.If Stentor coeriilens is A C c« c * FlG. 7. -Stentor cceruleus.A. Normal, fully expanded individual.A 1 .Same contracted.Line a-a indicates where it was cut in two.B, C.