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

BOOK 4

Worms, polyps, sponges, and so forth, are capable, we have seen, of total regeneration. Fragments of the body, torn off, can build up a whole new individual. It is evident enough that this power is related to sexless reproduction ; both are aspects of a tendency which primitive living matter has of rebuilding and reorganizing itself. As we pass the Animal Kingdom in review, turning from the simpler to the more complex, we see that the abilities to regenerate and to reproduce asexually fall off as the body gets more elaborately specialized. A newt or a lobster can regenerate limbs, but not much more than that. We ourselves can regenerate very little. The healing of a wound, the growth of new skin over a cut, the setting of a broken bone, the repair of a torn muscle or tendon—these are the last vestiges in our own bodies of this strange regenerative capacity. This is all that remains to us of the reproductive power that can make the severed flatworm into two individuals.

§ 8 Grafts and the Chimera

Man, we noticed, can do curious things with the normal rhythm of life. He can imitate fertilization and sexless reproduction. We turn now to an even more striking interference—to the mixing together of organisms of different kinds to form one.

It is well known that branches of plants can be grafted on to other plants, and that they will take and ‘“‘ become one flesh.” But grafting is possible with animal tissues, too.

John Hunter in the eighteenth century succeeded in grafting a cock’s spur and a piece of chick’s leg into another fowl’s comb, and had the satisfaction of seeing them continue to live and grow. Almost any organ of an embryo or any young and soft-bodied animal can be grafted anywhere else on the anatomy of the same or another creature of the same kind, and will “ take ”’ and grow. A tadpole’s growing limb or the regenerating leg-bud of a newt will grow after grafting on to the head or the flank, and will sprout out toes and turn into a wellformed leg quite happily. An eye-rudiment taken from one salamander embryo and stuck on to the flank of another grows into a normal-looking eye (though, of course, the animal can never see with it) and will even change its colour and shape, as do the animal’s own eyes when the host changes from tadpole to Jand-salamander. And

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CHAPTER 2

with very young embryos, whole regions, or even whole animals, can be grafted. Double tadpoles stuck back to back or belly to belly have been artificially produced, and will feed and grow and turn into double frogs. Moths too can be grafted while in the chrysalis stage, and each part will go on growing in its appointed way. By this means the most bizarre monsters can be created, such as two tail-ends joined in the middle, or a tandem moth with four pairs of wings.

The intimacy of union effected by grafting is shown very vividly by the mutual reaction of scion and stock in grafted plants. Identical apple-stems, for example, grafted on to different root-stocks behave very differently. For one thing, the colour and size of the scion’s apples are modified by the root-stock. On one stock, the scions show quick growth but fruit sparingly and late; on another they fruit early and well but stay small ; and some root-stocks seem to promote both rapid growth and heavy fruiting. The degree of the stock’s influence in these cases seems to depend on the place of grafting. The more of the stock is left above-ground, the greater its influence. These and other mysteries are being investigated by such research stations as that at East Malling in Kent, and the results are already proving of great commercial value by helping applegrowers to standardize their crops.

But the most extraordinary facts concern the making of one individual organism out of two; and the manufacture of compound creatures or chimeras. To make two individualities coalesce into one, it is easiest to work with eggs or very early embryos. For instance, if the fertilized egg of a newt is removed from its enveloping membrane, it elongates into a dumb-bell shape at its first division. If one such dumb-bell is lifted and placed across another, the cells adhere and the united mass may turn into a single but double-sized embryo. Even if the eggs come from different species of newt, the united pair may be reorganized into a single whole, which in this case merits the title of chimera, with its four quadrants alternately made of material from the one and the other species.

In making graft-experiments with young frog or newt embryos, it is also possible to unite bits of different species. In this way compound tadpoles, made of the front half of one kind of frog-embryo and the hind half of another, have been put together, and even reared through their transformation to become young frogs, which seemed in no