[ExI] The metacell uses a period 184 tractor beam, which acts as a clock.

[Eugen Leitl]

(why, young man, it's cells all the way down)

http://www.jwz.org/blog/2012/05/turtles-all-the-way-down-or-gliders-or-glider-turtles/

The metacell uses a period 184 tractor beam, which acts as a clock. It pulls
a block downwards by eight cells per impact, releasing a glider in the
process. Some of the gliders are utilised; the rest are eaten. When the block
reaches the base, it is restored at the top to begin the cycle again. Period
46 and 184 technologies (which are compatible) are used extensively
throughout the configuration.

The rule is encoded in two columns, each of nine eaters, where one column
corresponds to the 'Birth' rule and the other corresponds to 'Survival'. The
nine eaters correspond to the nine different quantities of on cells (0
through 8). The presence or absence of the eater indicates whether the cell
should be on in the next meta-generation. The state of the eater is read by
the collision of two antiparallel LWSSes, which radiates two antiparallel
gliders (not unlike an electron-positron reaction in a PET scanner). These
gliders then collide into beehives, which are restored by a passing LWSS in
Brice's elegant honeybit reaction. If the eater is present, the beehive would
remain in its original state, thereby allowing the LWSS to pass unaffected;
if the eater is absent, the beehive would be restored, consuming the LWSS in
the process. Equivalently, the state of the eater is mapped onto the state of
the LWSS.