FIG. 4: 20 b 0 50 100 150 200 250 Time (days) 2 4 6 8 Average increase in degree 0 50 100 150 200 250 Time (days) 50 100 150 200 Number of new links Number of nodes that at time  t at time  t create new links d c 0 5 10 15 20 l$_{N}$ 1 200 400 600 800 # cell types a 0 50 100 150 200 250 Time (days) 200 400 600 800 # cell types all embryo cells # non-surviving cells # surviving cells