REGENERATION IN ANIMALS. Like every substance, the tissues of our body are subject to wear and tear. They would soon be useless if there were no possibility of repair. The skin would rub off, the nails grind down, the hair split and fall, the muscles and nerves be worn out by fatigue. As it is, these tissues are incessantly growing but in such unobtrusive ways that we do not think much about the matter and take this physiological regeneration for granted, without inquiring into its causes. It is only in certain cases, when our skin peels off in sheets or when we trim our hair after the fashion and cut our nails, not worn enough by contact, that we become aware of this growth. In some animals the skin is periodically shed in one piece. In such creatures, for instance crayfish and snakes, growth of the whole body continues as long as these moults or ecdyses occur. Here the connection between physiological repair and growth is dis tinctly seen. Not only the damage done to our body by use, but also accidental injuries are made good by natural means. If we cut our finger, it will soon heal, the damaged cells being replaced by new ones. Torn-off nails may also regrow, an example of accidental regeneration in man, which exceeds mere wound heal ing. The loss of a whole limb, on the other hand, is irreparable by regeneration in human beings but not in other animal types. If a newt loses a leg or tail it will grow again. Even heads can be regenerated by some creatures, which are able to survive the accidental loss of the anterior part of their bodies.
Popular Belief.—Long before modern times the possibility of cut-off parts being regenerated fascinated the imagination of mankind. In Greek mythology the Hydra serpent, eventually slain by Hercules, was supposed to be able to grow two heads instead of each one cut off. Possibly this myth is founded upon the fact that not infrequently two-headed snakes are found in nature. Now, although snakes are neither able to survive the severing of the head from the trunk nor to regenerate it, the hypothesis is well substantiated that these monstrosities owe their origin to a process of splitting and regeneration in the embryo. Be this as it may, the name of the mythological serpent "Hydra" was later given to certain freshwater animalcules, when they were detected in the time of the first microscopes. The plant-like aspect of these hydras suggested experiments on multiplying them from cuttings much as gardeners do with certain plants. These animals 'behaved like plants, each piece growing into a perfect specimen. Moreover, when split at the anterior end they proved true to their name and regenerated two heads from one piece.
The occurrence of accidental regeneration was in some cases well known in ancient Greece. The Sauroktonos represents a youth in the characteristic position occupied by boys in Greece and Italy even to-day, when trying to catch lizards with a grass noose. This statue shows that from olden times the ways of lizards were known to their persecutors, and the easy breakage of the lizard's tail and regenerating individuals with tails in different stages of repair must have been observed. More than two cen turies ago a scientist travelling in Portugal recorded that many lizards lose their tails in this way and suggested that the not in frequent duplicities and triplicities of tails in that country might be of regenerative origin, due to complicated wounds; a sugges tion since verified by various experiments. Popular belief in the regenerative powers of the indigenous animals has not always proved reliable. In some parts of Italy the custom of eating the hind legs of frogs has led to the barbarous procedure of removing the legs from the living animals and returning the latter to the pond on the assumption that the legs will be regenerated. This erroneous idea seems to have arisen from the occasional observa tion of frogs with much smaller hind feet than usual. But ex tensive experimental work has shown that such individuals only arise when the legs are removed in the tadpole; legs removed after metamorphosis are never replaced. Another fallacy is the belief of Spanish fishermen that male fiddler-crabs robbed of the big claw they carry on one side grow a small one in its place, while the one on the other side enlarges correspondingly. In reality, the big claw, if lost, is replaced as such, the small one not changing its character. We shall see later on that in other Crustacea such transposition of chelae really occurs but only in special circumstances. Before experimental methods were applied, biological science made the same mistake in the case of the lob ster, transposition of the chelae being inferred to occur in the adult on account of stages found in nature showing a different degree of development in the claws. But recent experiment has proved that such reversal only occurs after removal of a claw in young stages : operations on the adult lead to a direct regeneration of the type of chela removed. Although the regeneration of claws in crustaceans was known to the ancient philosophers, experimental evidence was not obtained before the above-men tioned researches on microscopical animals led to methodical experiments with larger forms.