Figure 13.4a shows the urchin embryo as it appears six hours after development has begun (top left of diagram). This is the 16-cell stage, meaning that four rounds of cell division have already occurred (1 → 2 → 4 → 8 → 16). As development proceeds in the next four stages, both the number of cells and the degree of cellular specialization increases, until, at 55 hours, elements of the urchin skeleton come into focus. Figure
