How does the embryo develop?

To make a new person, a fertilised egg has to divide - making first two, then four, eight, 16, 32, 64, 128, 256 and eventually billions of cells. An embryo takes nine months to grow into a baby. To do this, the cells that make up the embryo have to move around, specialise, rearrange themselves and sometimes even commit suicide. How does each cell 'know' what to do, and what role do genes play in this process?

Colour-enhanced scanning electron micrograph of a human embryo at day 5 sitting on the tip of a pin.