Building a body is not simple. Fish, frogs and people all start from a single cell that becomes, seemingly against many odds, a highly organized, very complicated creature. Fertilized eggs split into two cells that become four, then eight, 16 and—within a matter of weeks—tens of thousands of cells. By this point the original spherical ball has rearranged itself into an elongated shape, bulging rounder and thicker at one end, with a shallow furrow running along its length. Soon another astonishing cellular ballet begins. The furrow deepens, and the cells that make up its walls begin to lean toward one another until they touch and stick together, forming a long, hollow tube that will eventually give rise to the brain at the bulging end and the spinal cord at the other.