“It’s amazing that [stem cells] have this capability,” says Jianping Fu, the University of Michigan professor in whose engineering lab Shao was a student. He says the emergence of something with an embryo’s shape, and some of its features, was “a complete surprise; I still can’t believe it. But it shows these cells remember what they are supposed to do.”

Scientists at Michigan now have plans to manufacture embryoids by the hundreds. These could be used to screen drugs to see which cause birth defects, find others to increase the chance of pregnancy, or to create starting material for lab-generated organs. But ethical and political quarrels may not be far behind. “This is a hot new frontier in both science and bioethics. And it seems likely to remain contested for the coming years,” says Jonathan Kimmelman, a member of the bioethics unit at McGill University, in Montreal, and a leader of an international organization of stem-cell scientists.

What’s really growing in the dish? There no easy answer to that.