Weissman opens his own talk with a call to arms. "I consider everything we do in this field to have a chance of advancing medical therapy." If embryonic stem cell research is delayed, he says, "some lives will be lost because of it. That's very important to know. I know it sounds slightly emotional, but I think that's the reality. While you wait, while you hold it back, while you have philosophical discussions, there are people who could have been saved."
Jon Cohen, a correspondent for Science magazine, is the author of Coming to Term: Uncovering the Truth about Miscarriage.
Photographer Mark Richards is based near San Francisco.