In World War II, Rita Levi-Montalcini sheltered in her childhood home in Italy and cobbled together a small research lab in her bedroom. With bombs falling around her, she used fertilized chicken eggs to understand a fundamental question: how does our nervous system make precise connections to its targets (muscles, etc) all over the body? She discovered that during embryonic development, we make far more nerve cells than we need. Those nerve cells grow out everywhere. Then, the many nerve cells that fail to connect to targets just die. It's an extraordinarily wasteful mechanism, but it's also an extraordinarily robust way to make precise connections.
Learn more: http://labs.bio.unc.edu/Goldstein/Nautilus.pdf
-Bob