The researchers hope these embryos, known as chimeras, could eventually help save the lives of people with a wide range of diseases.

One way would be to use chimera embryos to create better animal models to study how human diseases happen and how they progress.

Perhaps the boldest hope is to create farm animals that have human organs that could be transplanted into terminally ill patients.

But some scientists and bioethicists worry the creation of these interspecies embryos crosses the line. "You're getting into unsettling ground that I think is damaging to our sense of humanity," says Stuart Newman, a professor of cell biology and anatomy at the New York Medical College.