Fig. 3. Main layers of regulatory control within the PMC GRN. The earliest inputs into the PMC GRN are from maternal factors (blue box), followed by those of early specification genes (yellow box), and late transcriptional regulators (red box). As a consequence of these regulatory functions, two classes of terminal differentiation genes (green boxes) are activated: one that controls morphogenetic behaviors of the PMCs (morphoregulatory genes) and one that governs the synthesis of the endoskeleton (biomineralization genes). Regulatory interactions within and between levels are indicated by arrows.