http://www.sciencemag.org/news/2018/06/one-newly-discovered-cell-can-remake-whole-animal

Planaria are master regenerators, thanks a certain type of stem cell.

Though often no bigger than an apple seed, planaria are the envy of the animal kingdom. Cut them into a dozen pieces, and each piece will regrow into a full new worm—a remarkable feat of regeneration beyond the ability of most other animals. Now, researchers have pinpointed the cell—and a key protein—that kick-starts this process.