 Adequate philosophical models of the systematic dependence between genetic and physical difference makers in ontogeny need to account for how the temporal relations necessary for making causal claims are anchored in an external periodization used by developmental biologists. The imposition of different temporal scales can lead to distinct factors being significant or salient, which matters for ascertaining how different kinds of causes can be combined into integrated explanations. One possibility is to juxtapose these difference makers at distinct stages via experimental verification such that they exhibit productive continuity within the constraints of the external periodization (Love 2017b). This facilitates representing symmetry between causal factors because genetic difference makers can be placed before or after physical difference makers (and vice versa). Although this does not provide a way to combine causal magnitudes (as in vector addition from Newtonian mechanics), it offers an explicit strategy for assigning responsibility among different kinds of causes through the vehicle of temporal organization that goes beyond ranking difference makers. The periodization serves as a template from the practices of developmental biologists for providing wholeness or unity to the different modes of causation to yield a kind of integrated explanation of the morphology that results from a sequence of developmental processes.