 Next, consider the cellular-physical mechanism of branching morphogenesis, which refers to combinations of cellular proliferation and movement that yield branch-like structures in kidneys, lungs, glands, or blood vessels. There are many types of branching morphogenesis, but one primary mechanism is epithelial folding, which involves cells invaginating at different locations on a structure to yield branches (Davies 2013, ch. 20). Different cellular-physical mechanisms can produce invaginations that lead to branching structures (Varner and Nelson 2014): the constriction of one end of a subset of columnar cells in an epithelium (“apical constriction”); increased cell proliferation of one epithelial sheet in relation to another (“differential growth”); and compression of an epithelium leading to periodic invaginations (“mechanical buckling”). That different mechanisms can lead to the same morphological outcome means it can be difficult to discern which mechanism is operating in an embryonic context. Branching morphogenesis also exhibits all four of the shared elements of a mechanism. The parts are cells and tissues with activities or component operations (e.g., apical constriction, differential growth, mechanical buckling) being organized into patterns of interacting relationships (apical constriction leading to epithelial invagination) within a spatiotemporal context (in tracheal precursors within the Drosophila embryo around Stage 7 and 8). This organization produces a specific behavior or phenomenon (a set of branching structures—the trachea).