You're in a burning room with a wheelchair-bound adult and a freezer full of blastocysts. You can't save both the tiny embryos and the adult, so whom do you save?

I was asked this question by Hans S. Keirstead, an embryonic stem-cell researcher at the Reeve-Irvine Research Center in Southern California. He wanted to show that, when push comes to shove, all of us grade human life on a sliding scale. What he didn't realize was that he was talking to a woman who had once chosen the life of the embryo growing inside her over her own life as she had envisioned it.

Ever since speculation over the healing potential of human embryonic stem cells began circulating after scientists isolated them in 1998, I have wondered what I would do if scientists ever extracted a cure for my child's incurable disease from tiny human lives like the one I had cherished. I set out to find answers by talking to two stem-cell scientists about the ethics of their work. It was a remarkable entry into a world of half-truths, hesitancy, and awkward logic.

I couldn't suppress a slight smile when Dr. Keirstead met me outside his office barefoot and wearing frayed jeans. He is known, after all, as the hip, telegenic face of embryonic stem-cell (ESC) research. Keirstead catapulted out of obscurity when he and a colleague made paralyzed rats walk by injecting them with cells derived from human embryonic stem cells. For some time now, he has publicly contended that human trials will begin "next year," but he only recently published his findings, and other ESC scientists I spoke to were highly skeptical of this assertion. Keirstead was on the committee for Proposition 71—the initiative to spend $3 billion on ESC research in California—and ...