Models have emerged from physics on the laws of city growth, just like the laws on tidal waves and wildfires. In the 1930s Lewis Mumford had associated the growth of a big city with the spread of a microbial colony.
Just like the amoeboid it's failing to divide its social chromosomes and split up into new cells. The big city continues to grow by breaking through the edges and accepting its sprawl and shapelessness as an inevitable by-product of its physical immensity*.
Cairo's edges are cluttered lines that crawl into the Delta and, when the outlines of are drawn and compared with the shapes of microbial colonies, it becomes difficult to tell them apart.
- Philip Ball, Critical Mass, Arrow Books, 2004