The behavior of the code 20 cellular automaton from the top of the facing page for all initial conditions with black cells in a region of size less than nine. In most cases the patterns produced simply die out. But with some initial conditions, persistent structures are formed. Each initial condition is assigned a number whose base 2 digit sequence gives the configuration of black and white cells in that initial condition. Note that initial conditions 195 and 219 both yield the period 22 persistent structure shown on the next page.