Venturing deep into uncharted realms of ethics and medicine, a small biotechnology company said that its scientists had for the first time made human cells revert to the embryonic state from which all other cells develop by fusing them with cow eggs and creating a hybrid cell.

The company, Advanced Cell Technology of Worcester, Mass., said the method could eventually be used to grow replacement body tissues of any kind from a patient's own cells, sidestepping the increasing scarcity of organs available for transplant and the problems of immune rejection.

The technique is likely to concern and perplex ethicists because it involves the creation of an embryonic cell that is part human and part cow, consisting of a human cell's nucleus inside a cow egg whose own nucleus has been removed. The company said the hybrid cell quickly became more human-like as the human nucleus took control and displaced cow proteins with human proteins.

The technique involves creating an embryo of uncertain moral status, and one that crosses the barrier between humans and other species. Even though the hybrid is in the form of cells, not a whole organism, the concept of half-human creatures arouses deeps anxiety, as is evident from the unfriendly powers ascribed to werewolves, centaurs, mermaids, Minotaurs and other characters of myth and folklore.

"Many people are going to be horrified by this scenario," said Thomas Murray, director of the center for biomedical ethics at Case Western Reserve University in Cleveland and a member of the National Bioethics Advisory Commission. "This is the sort of thing that makes me very uncomfortable. I would like for all of us to have a breathing space here to articulate our moral concerns."

Genetic engineering could be wonderful. Or then again, maybe not. Consider the cautionary tale of the celery. In the mid-1980s, celery growers in the US introduced what they thought was a wonderful new strain. Highly resistant to insects, it promised to boost yields dramatically. There was just one small problem. People who handled the celery sticks began complaining of severe skin rashes. Dermatologists discovered that the celery was shedding psoralens, natural chemicals which become irritants and mutagens when exposed to sunlight.

Or take the once notorious American Lenape--or rather, don't. All seemed well with this hardy new variety of potato launched in the US and Canada in the 1960s. Then came the bitter truth. Biochemists discovered the source of the tuber's unusual burning flavor: dangerous levels of toxins called glycoalkaloids.

"Many nightmares predicted for genetically engineered crops have already happened," reflects Tony Conner of the New Zealand Institute for Crop and Food Research near Christchurch. It's just that "not many people noticed or cared" because they were the fruits of conventional breeding, not genetic manipulation.

A South Korean medical research team said Wednesday it has succeeded in cultivating a human embryo using human cells in one of the first cloning experiments of its kind, Reuters reported.

Researchers at the infertility clinic of Kyunghee University Hospital in Seoul said they had cultivated a human embryo in its early stages using an unfertilized egg and a somatic cell -- those that make up most of the body -- donated by a woman in her 30s. Lee Bo-yon, a researcher with the hospital's infertility clinic, said the human embryo in the Kyunghee University experiment was last seen dividing into four cells before the operation was aborted. Lee said the experiment was, to his knowledge, one of the first to use only human cells in a cloning experiment.

Scientists have taken an historic step forward in unravelling the mysteries of life. Researchers in the UK and the US have mapped all the genes in a small worm called Caenorhabditis elegans. It is the first animal for which this has been done. It has taken 15 years to complete and will have major implications for human health. Most people will never have heard of the creature which grows to about 1mm in length and lives in soil or among rotting plants, but it has much in common with man. Around 40% of the worm's genes - which hold all the instructions to build and maintain the creature - are also found in humans.

This means that a study of C. elegans will also reveal much about the biological processes inside humans. C. elegans may be a much simpler lifeform but it also begins life as a single, fertilised cell and undergoes a series of cell divisions as it grows into an adult animal. During this process, it also develops complex tissues and organ systems. It even has a nervous system that can detect odour, taste, and respond to temperature and touch.

Getting all this information has been a huge undertaking for scientists. In the early days, it was a very slow process. But the advent of special sequencing machines, which can work around the clock "reading" small sections of DNA, has allowed the genome to be mapped more quickly. The Internet has also proved to be an invaluable tool, enabling scientists to share data. These automation techniques will also speed-up the bigger and more important task of mapping the human genome, estimated to contain about 100,000 genes. 2003 is now the target date set for the completion of this project.

Completion of the worm genome is being hailed as a major scientific milestone. A spokesman said: "We have provided biologists with a powerful new tool to experiment with and learn how genomes function. We'll be able to ask and answer questions we could never even think about before."

Using a technique similar to the one that Scottish scientists used to clone Dolly the sheep, doctors in New York have for the first time transferred genes from an infertile woman's egg into another egg, fertilized it with sperm, and placed the resulting embryo in the womb in the hope of growing a baby.

The work breaks new ethical ground by being the first to mix significant amounts of DNA from two women's eggs into a single egg. That means any resulting child will have two genetic mothers--although one woman contributes vastly more than the other and so will clearly be the dominant biological mother.

"For the child and family, it raises questions of what bloodline means and what kinship means," said John Robertson, a professor of law and bioethics at the University of Texas in Austin.

An American scientist is forcing the world to confront the stark ethical choices involved in gene therapy, which could eradicate terrible diseases from a fetus, but would also make possible the first designer babies.

French Anderson, a pioneer of human gene therapy at the University of Southern California, Los Angeles, believes he could cure two inherited diseases. One, alpha-thalassaemia, is a blood disorder which kills the baby in the womb or shortly after birth.

His revolutionary therapy involves inserting a healthy gene to replace a damaged one. His technique is to introduce a virus carrying the new gene. But once the method is perfected, it could just as easily be used to exchange or add genes that determine physical appearance. Brown eyes could be swapped for blue, short stature for height. It might one day be possible to alter the character and the intellect of the unborn.

He says what he is doing is "a radical departure from anything that's ever happened before in medicine." Experiments that would change the genetic structure of an unborn human are banned in the U.S. and Britain at present, but the pressure for change, to eradicate distressing inherited medical conditions, is growing.

The Council for Responsible Genetics, a Massachusetts-based public interest group, condemned the proposal: "This is how it begins. Do we want a future in which babies are produced according to genetic recipes?"