In 1964 a British scientist, John Gurdon, performed what was to become a classic experiment in modern biology. Dr. Gurdon took the nucleus from a frog's intestinal cell and transferred it to an egg from which the natural nucleus had been withdrawn. The egg went on to develop into a normal adult frog, an exact genetic replica of its single parent. The experiment resolved a hundred-year-old controversy over how single cell develops into a multibillion-celled organism made up of thousands of different cell types.

But Dr. Gurdon's frog did something more, for it was the first cloned animal, and it raised for the first time the serious possibility that someday it might be possible to clone -- that is, to produce unlimited numbers of genetically identical offspring from the billions of identical nuclei present in one individual -- human beings.

The successful cloning of three mice, reported this week, is the next step. Although the experiments do not provide new intellectual insights, the carrying out of nuclear transplants in mammalian rather than amphibian cells represents a substantial technical achievement; amphibian cells are larger, more hardy in the laboratory and were thought to be biologically more flexible than mammalian tissue. The three cloned mice therefore bring the prospect of human cloning, with all its chilling overtones, a large step closer.

Nevertheless, cloning as anything more than a laboratory rarity is still far in the future. The key question is, how far? The remaining technical barriers are enormous, but biology has progressed faster than even the most visionary scientist would have guessed a decade ago, and no one could today hazard a confident guess as to when the remaining problems might suddenly be solved.

Ethical and social questions raised by scientific advances are notoriously difficult, if not impossible, to grapple with before the speculative theoretical matters they address become practical reality. The question that has troubled some biologists for years is whether genetic engineering -- including cloning, gene therapy and other new possibilities -- will raise social dilemmas on a par with those posed by the discovery of nuclear fission. When these possibilities become the reality, will society be any better prepared to deal with them than it was to deal with the ramifications of the bomb?