At first, Egli did not want me to watch him perform mitochondrial replacement on human eggs. He was afraid I would distract him. The eggs were precious: both challenging for researchers to obtain and a reminder of our fragile origins, and he was anxious not to slip and damage one. Several weeks after the F.D.A. meeting, he relented, on the condition that I camouflage myself among a cluster of CO2 tanks in the corner of the lab and not ask any questions. “If you look at what I’m doing,” he said, “there is really nothing to be afraid of, I think.”


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