General Knowledge
Why does cutting a worm in half not give you two worms?
- Only the head end can regrow, and not always
- It does give you two
- Both halves die
The answer is Only the head end can regrow, and not always. The head end may regrow a tail; the tail end has no brain and dies.
Because only one end has the equipment. An earthworm's head segment carries the brain, the mouth and the reproductive band, and it can often regrow a new tail; the tail end has none of those and cannot grow a new head, so it dies.
Even the surviving half is not reliable. How much it can regrow depends on how far back the cut is - lose too many front segments and the worm cannot feed while it repairs - and the regrown section is frequently shorter, with fewer segments than the original.
Some animals really can do it. Planarian flatworms cut into pieces regrow into complete individuals from remarkably small fragments, because they carry a large population of stem cells throughout the body; a cut across the head can produce a two-headed worm. Sea stars regrow arms and, in a few species, a whole animal from one arm with a piece of the centre. Understanding why humans cannot is a substantial part of regenerative medicine.