 A well-established molecular genetic mechanism is the initial formation of segments in Drosophila due to the segment polarity network of gene expression (Wolpert et al. 2010, 70-81; Damen 2007). By Stage 8 of development (~3 hours post-fertilization), Drosophila embryos have 14 parasegment units that were defined by pair-rule gene expression in earlier stages. The transcription factor Engrailed accumulates in the anterior portion of each parasegment. This initiates a cascade of gene activity that defines the boundaries of each compartment of cells that will eventually become a segment. One element of this activity is the expression of hedgehog, a secreted signaling protein, in cells anterior to the band of cells where Engrailed has accumulated, which marks the posterior boundary of each nascent segment. This, in turn, activates the expression of wingless, another secreted signaling protein, which maintains the expression of both engrailed and hedgehog in a feedback loop so that segment boundaries persist  (Figure 4).  The segment polarity network exhibits all four of the shared elements of a mechanism. It is constituted by a number of parts (e.g., Engrailed, Wingless, Hedgehog) and activities or component operations (e.g., signaling proteins bind receptors, transcription factors bind to DNA and initiate gene expression), which are organized into patterns of interacting relationships (feedback loops, signaling pathways) within a spatiotemporal context (in parasegments of the Drosophila embryo, ~3 hours post-fertilization) so as to produce a specific behavior or phenomenon (a set of distinct segments with well-defined boundaries).