### Planarian Flatworm
Schmidtea mediterranea
Chop it into microscopic pieces, and every single fragment will grow into a perfect, living clone.
The Story
Meet the Planarian Flatworm (Schmidtea mediterranea), a squishy creature with practically zero physical strength or armor. If you cut a standard comic book superhero in half, you get a tragedy. If you cut a planarian in half, you get two planarians.
But it gets much weirder than just splitting down the middle. In classic experiments, scientists discovered that you can take just 1/279th of a planarian's body—a microscopic fragment basically the size of a speck of dust—and it will survive to completely rebuild itself.
Over the next one to two weeks, that speck doesn't just close a wound; it regrows absolutely everything. It builds brand-new eyespots, a fresh pharynx, and a totally complete central nervous system from scratch. It is the ultimate biological "undo" button.
How It Works
- The Neoblast Army: In a human, "pluripotent" stem cells (blank-slate cells that can turn into any tissue) exist only before we are born. But an adult planarian is packed with them. These cells, called neoblasts, make up a staggering 25% to 30% of the worm's entire body. - Building the Blastema: When the worm is cut, neoblasts migrate to the injury site within hours. They divide rapidly to form a construction zone of growing tissue called a blastema. - Chemical Compasses: How does a tiny chunk of worm know which end gets the brain? It uses complex chemical signals (like the Wnt signaling pathway) as a GPS to tell the blastema exactly where to put the new head and tail.
