The seven-cell embryo was implanted in the mouse uterus and grew successfully to term. This part of the procedure is known to work with humans too, because it is the basis of a well-established test known as preimplantation genetic diagnosis. In the test, one cell is removed from each of a set of embryos and tested for any of 150 genetic defects, giving the parents the choice of implanting an embryo that is disease free.

Dr. Lanza's technique is likely to be welcomed by many in the middle of the debate, although it has not won over the United States Conference of Catholic Bishops. Richard M. Doerflinger, its deputy director for pro-life activities, dismissed the technique, saying that preimplantation genetic diagnosis itself is unethical.

The technique "is done chiefly to select out genetically imperfect embryos for discarding, and poses unknown risks of future harm even to the child allowed to be born," Mr. Doerflinger said in an e-mail message.

Only a procedure that generated embryonic stem cells without creating or destroying embryos "would address the Catholic Church's most fundamental moral objection to embryonic stem cell research as now pursued," Mr. Doerflinger said in testimony last December to the President's Council on Bioethics.

Senator Sam Brownback, a Kansas Republican and a leading pro-life advocate did not return a call to his office. Edmund D. Pellegrino, the new chairman of the President's Council on Bioethics, said through a spokeswoman that he had no comment.

But Markus Grompe, a leading stem cell scientist and a Roman Catholic who supports the church's teaching on the unacceptability of destroying embryos, praised the Lanza approach, provided that the extracted cell could not develop into an embryo by itself. "I find it clearly less objectionable than the outright destruction of the embryo," said Dr. Grompe, who studies liver stem cells at the Oregon Health and Science University.

In response to Dr. Grompe's reservation, Dr. Lanza said individual human blastomeres, as the cells are known at this stage, had never been shown to create viable embryos.

If Dr. Lanza's technique proves to work in humans, it could do more than just provide researchers with a new source of cells. It might allow every child born through preimplantation genetic testing to have its own line of embryonic cells stored for the future. The blastomere removed at the eight-cell stage could be allowed to divide, with one cell being used for genetic testing and the other for growing a culture of perfectly matching embryonic stem cells.

The cells would be available throughout the child's life for the kind of tissue and organ repair that it is hoped stem cells will one day provide. In many of the degenerative diseases of old age, from heart attacks to Parkinson's, the body loses vital cells and fails to replace them, an omission that could perhaps be overcome if embryonic cells like those present at the beginning of life were available to generate replacement cells artificially.