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Image is of a black and white cow with yellow tags on its ears facing towards the screen.

It was a cool morning at the beef teaching unit in Gainesville, Florida, and cow number #307 was bucking in her metal cradle as the arm of a student perched on a stool disappeared into her cervix. The arm held a squirt bottle of water.

Seven other animals stood nearby behind a railing; it would be their turn next to get their uterus flushed out. As soon as the contents of #307’s womb spilled into a bucket, a worker rushed it to a small laboratory set up under the barn’s corrugated gables.

“It’s something!” said a postdoc named Hao Ming, dressed in blue overalls and muck boots, corralling a pink wisp of tissue under the lens of a microscope. But then he stepped back, not as sure. “It’s hard to tell.”

The experiment, at the University of Florida, is an attempt to create a large animal starting only from stem cells—no egg, no sperm, and no conception. A week earlier, “synthetic embryos,” artificial structures created in a lab, had been transferred to the uteruses of all eight cows. Now it...