REGENERATION

Although a few cases of regeneration were spoken of by Aristotle and by Pliny, the subject first attracted general attention through the remarkable observations and experiments of the Abbé 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 pieces; for it was generally known that pieces of a plant made a new plant, but if an animal were cut into pieces, the pieces died. Trembley found that the polyp, if cut in two, produced two polyps. Logically, he should have concluded that the new form was a plant; but from other observations, as to its method of feeding and of movement, Trembley concluded that the polyp was an animal, and that the property of developing a new organism from a part must belong to animals as well as to plants. "I felt," he says, "strongly that nature is too vast, and too little known, for us to decide without temerity that this or that property is not found in one or another class of organized bodies." Trembley's first experiments were made in 1740, and the remarkable results were communicated by letter to several other naturalists. It came about in this way that before Trembley's memoir had appeared, in 1744, his results were generally known, and several other observers had repeated his experiments, and extended them to other forms, and had even published an account of their own experiments, recognizing Trembley, however, as the first discoverer. Thus Réaumur described, in 1742, a number of other forms in which regeneration takes place; and Bonnet, in 1745, also described some experiments that he had made during the four preceding years. Widespread interest was aroused by these results, and many different kinds of animals were experimented with to test their power of regeneration. Most important of these new discoveries were those of Spallanzani, who published a short preliminary statement of his results, in 1768, in his Prodromo.{2} 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 a new head, and a two-headed hydra results. If each of the new heads is split again, a four-headed hydra is produced; and if each of the four heads is once more split in two, an eight-headed hydra is formed. A hydra of this kind, in which seven heads had been produced in this way, is shown in Fig. 1, A. Each head behaves as a separate individual, and all remain united on the same stalk. If the foot-end of a hydra is split, a form with two feet is produced.