Fagan (2012, 2013) emphasizes the interdependent relationship between parts of a mechanism. Components in a mechanism, she points out, often form a more complex unit by virtue of the individual properties that unite them—their “meshing properties”; the complex unit then figures into the mechanism’s behavior. This interdependent relationship—jointness—is exemplified by the lock-and-key model of enzyme action. Fagan applies this notion to research on stem cells (Fagan 2013) but argues that it is a general feature of experimental biology (Fagan 2012).