The Universal Constructor
What is it?

Similar concept to the Universal Turing Machine

A machine embedded within a CA which, when fed a "tape" containing
suitable instructions, can construct any other cell assembly (hence the
term Universal Constructor)

It follows from this that the Universal Constructor is also
capable of Universal Computation

Consists of 3 basics actions:

Translating the tape

Copying the tape

A controller to switch between
translating and copying

It can be fed instructions to build itself, and is thus capable
of selfreplication
Problems?

Von Neumann designed it to be able to withstand some mutations,
and thus be evolvable,
but there are some practical difficulties with this...

It's huge! (29 states per
cell, 1000s of cells!)
Implementation

Von Neumann designed a Universal Constructor, but didn't
implement it

The first real implementation was completed very recently,
by Pasavento and Nobili [see U. Pasavento,
"An Implementation of von Neumann's SelfReproducing Machine", Artificial
Life 2: 337354 (1995)]

An online
demo is available