In a lab in MIT, a flatworm is dying. It’s a planarian – a simple animal that is normally very difficult to kill. Planarians are masters of regeneration; whole animals can be reborn from small clumps of tissue. If you cut one in half, it will simply grow into two planarians. But this animal has been bombarded with high doses of radiation that have wiped out its ability to regenerate. Slowly, its cells are bursting apart. With no new ones to replace them, the planarian has a few weeks to live.

But Daniel Wagner and Irving Wang are about to save it, in a fashion. They transplant one special cell from a donor planarian into the terminal individual’s tail. The cell starts to divide. It produces skin, guts, nerves, muscle, eyes and a mouth.

As the planarian dies from the head backwards, the transplanted cells spread from the tail upwards. At its worst, the animal is a stunted mass with no discernible head. But two weeks after the transplant, it has completely regenerated. A new planarian has risen, phoenix-like, from the ashes. Its entire body is now genetically identical to the ...