"The age of the laboratory-bred human being began with Louise Brown, who was conceived in a petri dish and born to an English couple in 1978. Since then, the world has seen other babies born after fantastic embryonic adventures — in vitro conception like Brown's, transfer from the womb of one woman to a more receptive womb in another, frozen suspension inside a tank of frigid liquid nitrogen. The survival of these embryos in unfamiliar — and sometimes inhospitable — surroundings suggests that hardy embryos would also survive if scientists were to wield their microsurgical blades on them, to create twins or even more copies from a single human embryo.

      "More than other techniques for embryo manipulation now in use, twinning raises a troublesome question: How far should man go in determining his heredity? Because twinning would not correct any genuine medical problem, unlike in vitro fertilization or embryo transfer, it strikes some scholars of bioethics as frivolous at best. For others, the prospect of its widespread use raises the specter of eugenics — the engineering of human heredity — and the dark memory of the Third Reich's efforts to "improve" the race.

      "The technique introduces few new problems of legal and medical policy, since it falls into the category of embryo manipulation, which also includes in vitro fertilization and embryo transfer. When those techniques entered the medical arena several years ago, there was protracted public debate about their morality. The debate quieted down as their use became more commonplace and produced healthy babies, but flared anew in June with the revelation that a Los Angeles couple who died in a 1983 plane crash left behind two frozen embryos in an Australian medical center. The embryos, frozen since 1981, may not survive thawing, but a government ethics committee there must now decide their fate: whether they should be destroyed, or perhaps donated to surrogate mothers, and what legal rights — if any — they might have as heirs to the couple's estate, which may be as large as $7 million.

      "In the United States, decisions about using either in vitro fertilization or embryo transfer are largely left to the doctor and patient. No federal agency regulates them. In fact, the law has basically encouraged such measures when married couples have difficulty conceiving.

      "Twinning does not affect the genetic make-up of the embryo. Each half retains its full complement of chromosomes from both parents. The problems with embryo splitting arise from its ability to create up to four copies of a single embryo, perhaps years apart. It is, in essence, cloning.

      "At its simplest, embryo splitting could allow people to choose to have identical twins, triplets, or quadruplets. The procedure could even produce mutigenerational twins. While whole embryos can be kept alive if they are carefully frozen and thawed, halved embryos rarely survive these temperature swings, because ice crystals can form inside their broken sacs. But if freezing halved embryos could be perfected, one half could be implanted several years after the first to produce twins of different ages. If a girl were born from one half and the other half were frozen until she reached puberty, she could give birth to her own twin.

      "The first researcher to attempt implantation of a split human embryo would have to clear the experiment with his university or hospital review board. These boards, required by the National Institutes of Health, must rule on any research involving people. Some observers suggest that embryo-splitting research might encounter its only major hurdle here. If the purpose of twinning were to correct or circumvent infertility, they say, it would probably face little resistance — but that is not its function. Boards could ask what legitimate reason there could be for producing twins. "To do such an experiment just to see if it could be done would seem to be wrong — a form of mischievous research," says Daniel Callahah of the Hastings Institute, a research center in New York that studies questions of scientific ethics.

      "In a general sense," says Richard Noble, who teaches bioethics at the University of Michigan, "the issues brought up by twinning are already upon us. With amniocentesis and selective abortion we're already deciding that some children are acceptable and some are not. We're already deciding what we want our children to be."

      "How far should human beings go in trying to determine the genetic make-up of the species? On one side of the argument are those who endorse a technology that enables human beings to control their environment — in this case their genetic heritage. These people do not admit the possibility that governments, for example, might seed poor women with embryos from a pool of elite parents. On the other side are those who think that it interferes with God's plan, or it is bound to go awry, considering man's demonstrated inability to anticipate the unintended consequences of his acts. "People are afraid that our moral and ethical ability to judge has been far outpaced by our research," Noble says. "This gives us a good reason to go slowly and cautiously."

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