At the core of Heinz Werner’s concept of development is what he called “the genetic principle of spirality:” over the course of ontogenetic development, lower levels, processes, and functions do not disappear, and can even resurface again under specific conditions, normal and pathological. Werner’s work with Bernard Kaplan on symbol formation is a primer on this idea. This paper examines the idea of spirality and develops the notion of dynamic coexistence that can clarify the issue of directionality of development; that is, what is the general trajectory or ground plan that development assumes. Directionality is discussed in terms of the organism-in-environment unfolding over time as the unit of developmental analysis. Thinking on this issue has proceeded from the nature–nurture debates, to recognition of the interaction of external and internal processes, to transactions between the organism and the environment. The idea of dynamic coexistence is developed on this foundation. In the context of Werner and Kaplan’s work, dynamic coexistence represents the syncretic nature of processes and levels of organization: they are neither innately fused nor organized. Instead, the antithesis between fusion and differentiation is resolved only in concrete, context-bound conditions, even as development is generally oriented towards differentiation in maturity. Idiographic science, with its orientation towards generalizing from the unique and its attention to intra-individual variation over time, is discussed as the adequate methodological orientation for Wernerian developmental analysis.