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It was day 13 in a set of experiments in Ali Brivanlou’s laboratory and he had an agonizing task ahead. His team of developmental biologists had thawed dozens of human embryos, placed them into individual culture dishes and watched them grow through the earliest stages of development — something that only a handful of researchers worldwide had ever seen. But he knew that it had to end.

The embryos would soon bump up against the 14-day rule, an international consensus that human embryos should be cultured and grown in the lab only until 14 days post-fertilization. Day 14 is roughly when the primitive streak appears, a structure that marks the point at which the embryo sets up the body axes, and begins to distinguish head from tail and left from right.

“It was one of the toughest decisions I’ve had to make in my life, but it was time to stop this experiment,” says Brivanlou, recalling the 2015 research in his lab at the Rockefeller University in New York City. On day 13, the team peeled the embryos from the... see more