At what point should a scientist stop doing experiments on a human embryo in a Petri dish?

For decades, the answer has been clear: 14 days after fertilization. The cutoff is protected by law in 12 countries, including Canada and the United Kingdom, and by scientific guideline in five others, including the United States.

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Many ethicists aren’t convinced, though. To Stanford Law School’s Hank Greely, extending the 14-day rule is a slippery slope: “I don’t know where you stop.”

“I do know that I would feel very concerned about a 20-week fetus being used as an experimental object, because it’s too damn close to being a baby,” he said. “And people should not be treated as objects.”