Rules for gut growth (see Fig. 4g) Field elongation rule, whereby gut mesoderm agents proliferate and daughter agents are placed longitudinally (ie. proximo-distally). This is modeled in CA by randomly selecting a mesoderm agent in each row to divide with daughter agents placed adjacent in the same row, thereby lengthening the row by one agent. Carriage rule, whereby an ENC agent occupying a mesoderm agent site which moves due to a mesoderm proliferation event, is carried distally with that mesoderm agent.