Cellularity is one of the fundamental properties of organic life, and can be recognized in the fossil record as far back as 3.6 billion years. The cell is the original individual, with the cell membrane defining its limits and preserving its chemical integrity. An analog to the cell membrane is probably needed in digital organisms in order to preserve the integrity of the informational structure from being disrupted by the activity of other organisms.
The need for this can be seen in AL models such as cellular automata where virtual state machines pass through one another [48], or in core wars type simulations where coherent structures that arise demolish one another when they come into contact [68,69]. An analog to the cell membrane that can be used in the core wars type of simulation is memory allocation. An artificial ``cell'' could be defined by the limits of an allocated block of memory. Free access to the memory within the block could be limited to processes within the block. Processes outside of the block would have limited access, according the rules of ``semi-permeability''; for example they might be allowed to read and execute but not write.