Oster and his colleagues followed the same strategy we considered earlier in this chapter for a computer
simulation of starlings flocking. Instead of programming the behaviour of a whole blastula, they programmed a single cell. Then they 'cloned up' lots of cells, all the same, and watched to see what happened when those cells got together in the computer. When I say they programmed the behaviour of a single cell, it would be better to say they programmed a mathematical model of a single cell, building into the model certain known facts about a single cell, but in simplified form. Specifically, it is known that the interiors of cells are criss-crossed by microfilaments: sort of miniature elastic bands, but with the additional property that they are capable of active contraction, like twitching muscle fibres. Indeed, the microfilaments use the same principle of contraction as muscle fibres.* The Oster model simplified the cell down to two dimensions for drawing on a computer screen, and with only half a dozen filaments, strategically placed in the cell, as you see in the diagram above. In the computer model, all the microfilaments were given certain quantitative properties with names that mean something to physicists: a 'viscous damping coefficient' and an 'elastic spring constant'. Never mind exactly what these mean: they are the kinds of things physicists like to measure in a spring. Although it is probable that in a real cell many filaments would be capable of contraction, Oster and his colleagues simplified matters by endowing only one of their six filaments with this capacity. If they could get realistic results even after throwing away some of the known properties of a cell, it would presumably be possible to get at least as good results with a more complicated model that kept those properties in. Rather than allowing the one contractile filament in their model to contract at will, they built into it a property which is common in certain kinds of muscle fibre: when stretched beyond a certain critical length, the fibre would respond by contracting to a much shorter length than the normal equilibrium length.
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