The embryo builder: This stem-cell scientist is creating synthetic embryos from stem cells.

Another policy under pressure is the “14-day rule,” a widely employed convention that natural embryos should not be grown longer than two weeks in the lab. Though it’s a mostly arbitrary stopping point, it’s been convenient for laboratory scientists to know where their limit is. But that rule isn’t being applied to the embryo models. For instance, even though the United Kingdom has a 14-day rule enshrined in law, that legislation doesn’t define what an embryo is. To scientists working on…