The science of life : fully illustrated in tone and line and including many diagrams

HOW INDIVIDUALS ORIGINATE

reorganize itself into hundreds of new and perfect organisms, is indeed hard to destroy. In the hydroid Clava, which normally has the sexes separate, it has been found possible to make hermaphrodite individuals ; such will sometimes grow out of the celldébris formed when a male and a female individual are sieved and dissociated together. Artificial reproduction is possible even in land vertebrates, though only in their earliest stages. A fertilized newt’s egg, up to the time when it forms definite celllayers, can be constricted into two parts by a fine hair, and each of the two halves may survive and grow into a whole newt : we can produce artificial twins. If we could but get at the earliest stage of our own development, doubtless we could equally well persuade a human egg to do at our will what it occasionally does at its owndivide into two and give rise to identical twins.

A Note on the Regeneration of Lost Parts

We turn now to a series of phenomena which are not in themselves reproductive but which are very evidently related to the reorganization of detached bits of one individual to form others, which is the essential reproductive process. It is an all-too-familiar fact that if we lose a limb or even a tooth or finger, we cannot replace it ; and all the creatures with whom we have everyday dealings suffer the same limitations. In fact, Mr. Everyman would be likely to assert that the capacity for regeneration was one of the points which separated animals from plants. This was firmly believed in the eighteenth century; and when the Abbé Trembley discovered the little green polyp Hydra, and was at a loss to know whether it was an animal because it moved or a plant because of its being fixed and green and flower-shaped, he used regeneration as a test. If he cut it in half and it regenerated, it would be a plant; if it did not, it would be an animal. He cut it in half, and both halves regenerated into perfect little wholes. However, common-sense prevailed over preconceived theory. He had seen the Hydra capture living prey, swallow and digest it. He pronounced it an animal, but an animal which could regenerate.

Starting from this discovery, biologists eagerly tested all kinds of creatures for their regenerative capacities, and found that regeneration is by no means a rare or

isolated phenomenon in animals, that worms can grow new heads or bodies, lobsters and crabs replace a missing leg or feeler, and even air-breathing vertebrates such as newts regenerate limbs and tail.

Regeneration in action still remains one of the most striking spectacles which biology can offer. A good-sized lobster loses its claw ; and at successive moults we see a little bud of tissue grow on the stump, take shape, become formed into a miniature new claw (still wholly useless for the big creature on which it is growing), and enlarge step by step until the animal is once again completely equipped. A newt has a leg bitten off ; it, too, will bud out a whitish lump from the cut surface, the bud will elongate and constrict itself into joints, bones and muscles will form inside it, and toes sprout out at its end, until the newt has achieved what is impossible to us lords of creation. It will restore just what was lost, and no more: if only the hand had been bitten off, the bud will turn into a new hand ; if the bite was at the shoulder, into a whole new limb.

We take a planarian worm (Fig. 92), and cut a small oblong out of its body. The fragment cannot eat, for it has no mouth. And yet it throws out an army of new and active cells on both its new frontiers, and these, dividing and growing and differentiating at the expense of the rest, form themselves into a new head and hinder end. At first these new parts are too small for the body; and accordingly remodelling goes on not only in these new-produced regions, but in the original fragment as well. They grow, it shrinks ; _and both they and it change shape, until in place of the original helpless fragment there is a new and well-proportioned little flatworm (Fig. 161). .

Regeneration is so queer and unfamiliar a process to the human investigator that it was thought at one time to be a special protective device evolved by creatures which are particularly exposed to injury. It was supposed, for example, that newts, being peculiarly liable to have their limbs bitten or torn off, had_ therefore evolved this wonderful aptitude for re-growth in order to save themselves from extinction. But, as a matter of fact, regeneration is a very surprisingly widespread _thing. Moreover, it is apparently a primitive thing, a power possessed by all the simplest forms of life, and one which we have lost, not one that the newt and the lobster and the flatworm

have gained.

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