As biological research races forward, ethical quandaries are piling up. In a report published Tuesday in the journal eLife, researchers at Harvard Medical School said it was time to ponder a startling new prospect: synthetic embryos.

In recent years, scientists have moved beyond in vitro fertilization. They are starting to assemble stem cells that can organize themselves into embryolike structures.

…

Henry T. Greely of Stanford University was less optimistic. While it is important to have a discussion about Sheefs, he said, it may be hard to reach an agreement on limits as enforceable as the 14-day rule.

“Whether you could come to some consensus is really doubtful,” he said.