 Not all types of causal explanation involve an external periodization and there are other ways to combine causes in order to produce more integrated explanatory frameworks. One area where combined explanations for developmental phenomena are being analyzed pertains to mechanism descriptions and mathematical modeling in systems biology (Brigandt 2013; Fagan 2013). For example, Fagan (2013: ch. 9) shows how an integrated explanation emerges from a step-wise procedure that starts with a detailed description of a molecular mechanism followed by the formulation of an abstracted wiring diagram of component interactions, which is then translated into a system of equations that can account for changes in component interactions over time. Solutions to these systems of equations and a mapping of solutions for the interactions among components of the system onto the behavior of the overall system within a shared landscape representation more systematically explains cellular differentiation.