Evolution,

guidance but there is uncertainty about whether it is a recommendation or subject to the force of law. Still, the country’s scientists are outraged, with 122 signing a letter calling his work “crazy.” As Prof. Lovell-Badge puts it: “They’re worried about their labs closing down.” Meanwhile, nobody has seen a photograph of Lulu and Nana; Mr. He says he wants to guard their privacy but, until proof is presented, a hoax cannot be ruled out.

Not everyone is willing to condemn Mr. He. George Church, a professor of genetics at Harvard University most famous for wanting to resurrect the woolly mammoth, cautions that Mr. He has not caused known harm and likens the backlash to bullying.

He points out that a moratorium is not the same as a permanent ban, and contrasts this week’s controversy with the case of Jesse Gelsinger, a teen who died in 1999 during a gene therapy trial for a metabolic disorder.

Preparatory animal experiments had been done. “The most serious thing I’ve heard is that He didn’t do the paperwork right,” Prof. Church told the journal Science. “If . . . someone had been damaged, maybe there would be some point, like what happened to Jesse Gelsinger. But is this a Jesse Gelsinger or a Louise Brown event?”

Louise Brown, born in 1978, was the first test-tube baby, and perhaps the closest historic and scientific parallel to Lulu and Nana. She, along with an estimated 8 million other IVF babies, owes her existence to the controversial efforts of British pioneers Robert Edwards and Patrick Steptoe. Louise arrived first; societal acceptance of labmade babies came later.

Mr. He sees himself not as a rogue scientist but as a pioneer in the same tradition, describing Lulu and Nana in the video as the next step in IVF. “As a father of two girls I can’t think of a gift more beautiful and wholesome for society than giving another couple a chance to start a loving family,” he said. “We hope you have mercy for them.