Figure: 
: Another cycle for an eight cell ring twice
repeated.
Cycles of period four occur in a variety of forms during evolution
according to Rule 22, although only one of them makes its appearance in
the context of a sixteen cell ring (as a doubled eight cell ring), as
shown in Figure 
. They all evolve from a pair of cells with
small gaps, which can sometimes sum up to produce the same ring length
in different ways.