 “Normal development” is conceptualized through strategies of abstraction that manage variation inherent within and across developing organisms (Lowe 2015, 2016). The study of ontogeny in model organisms  (Section 4)  is usually executed by establishing a set of normal stages for embryonic development (see Other Internet Resources). A developmental trajectory from fertilized zygote to fully-formed adult is broken down into distinct temporal periods by reference to the occurrence of major events, such as fertilization, gastrulation, or metamorphosis (Minelli 2003: ch. 4; see  Section 1.2).  This enables researchers in different laboratory contexts to have standardized comparisons of experimental results (Hopwood 2005, 2007). They are critical to large communities of developmental biologists working on well-established models, such as chick (Hamburger and Hamilton 1951) or zebrafish (Kimmel et al. 1995): “Embryological research is now unimaginable without such standard series” (Hopwood 2005: 239). These normal stages are a form of idealization because they intentionally ignore kinds of variation in development, including variation associated with environmental variables. While facilitating the study of particular causal relationships, this means that specific kinds of variation in developmental features that might be relevant to evolution are minimized in the process of rendering ontogeny experimentally tractable (Love 2010).