North American scientists are considering whether to implant stem cells from a human embryo into a mouse embryo -- a controversial experiment that could result in a mouse-human mix reminiscent of mythological monsters. The chimera proposal, a chimera being a creature born of at least two different species, might, in theory, allow scientists to test if human embryonic stem cells do indeed have the power to grow into every tissue type in the body. But if it works, the experiment also raises the eerie, and ethically charged prospect of creating a mouse with troubling human features -- anything from a human brain to human sperm and eggs. "If it really did work, then you'd have this very uncomfortable creature to work with," said Janet Rossant, a leading Canadian stem-cell researcher and an opponent of the experiment. "Do you generate a human brain in a mouse . . . where do you draw the line?" Dr. Rossant, a senior scientist at Toronto's Mount Sinai Hospital, was among a select group of researchers and observers at a closed New York meeting on Nov. 13, where the chimera experiment and other topics were debated. Sponsored by Rockefeller University and the New York Academy of Science, the day-long meeting was held to discuss ways to evaluate the power and versatility of the lines of human embryonic stem cells now being grown in labs around the world. Dr. Rossant said that to gauge the quality of stem cells from a mouse embryo, researchers insert the cells into the embryo of another mouse as the gold standard measure. If the mouse stem cells, which are tagged with a fluorescent marker, are truly pluripotent (have the ability to grow into every tissue type), they can be detected as blood, bone, muscle and even brain cells all over the mouse. But to conduct the same experiment with human embryos would be clearly unethical. As a result, she explained, researchers at the New York meeting raised the possibility of injecting human embryonic stem cells into an early mouse embryo. Scientists have for years successfully inserted human genes into mouse DNA to study certain diseases. They have also fused mouse cells with human cells. But they have never created a mouse carrying human cells, each of which contains a full complement of human genes, said Ronald Worton, head of Canada's Stem Cell Network and the Ottawa Hospital Research Institute. The Canadian Institutes of Health Research, which distributes federal funds for medical research, had envisioned the prospect of chimera experiments. CIHR president Dr. Alan Bernstein noted yesterday that no Canadian scientist receiving CIHR funds would be permitted to conduct such an experiment. "We don't even really know what happens when you mix two different species like this," he said, "Are you now going to have a mouse walking around with human sperm -- what would the public reaction be to that?" Aside from moral concerns, Dr. Rossant said the chimera experiment would be scientifically hamstrung. If it failed, she explained, researchers would never know if the human embryonic stem cells were faulty, or if they were unable to survive in the mouse environment. Dr. Worton agreed, noting that a human develops over nine months and a mouse in 20 days. Dr. Rossant suggested that human embryonic stem cells could instead be grown and tested in culture dishes, or in tissue grafts using animal models, without the need for creating a chimera. Dr. Worton also questioned the timing of the proposal and the fears it will feed among those who oppose embryonic stem cell research. Research with human embryonic stem cells is controversial because a human embryo is destroyed in the process of harvesting the prized stem cells. "We're all responsible citizens, not Frankensteins, and we're trying to explain that we're interested in doing necessary experiments," Dr. Worton said, "and it seems to me that to begin talking about creating a human-mouse [mix] is waving a red flag unnecessarily."