Pig embryos that had been injected with human stem cells when they were only a few days old began to grow organs containing human cells, scientists reported on Thursday, an advance that promises — or threatens — to bring closer the routine production of creatures that are part human and part something else.

These human-pig “chimeras” were not allowed to develop past the fetal stage, but the experiment suggests such creations could eventually be used to grow fully human organs for transplant, easing the fatal shortage of organs: 120,000 people in the United States are waiting for lifesaving transplants, but every day two dozen die before they get them.

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But it’s one that bioethicists have warned about for at least a dozen years, since advances in stem cell biology made it easier to produce chimeras. The creation of “intermediate forms” of life is thought by some critics to “denigrate human dignity and blur the line between what is human and what is not, especially if you believe that we were created in the image of God,” said bioethicist and legal scholar Hank Greely of Stanford.

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Stanford’s Greely predicts that ethical concerns will abate. “In American bioethics, ‘cures’ is the trump card,” he said. “You play that angle” — such as by saying chimeras will provide transplantable organs to dying patients — “and, politically, you almost always win.”