 The process of advancing new conceptions of cells continues. We note just one example here. The pioneers of cell biology in the 1940s and 1950s embraced the machines in a factory metaphor, treating cell organelles as compartments in which different chemical reactions were catalyzed by the enzymes housed there. Once formed, the compartments, on this view, did not change—the crucial activities occurred within them. Over time, however, some researchers within cell biology began to identify cell structures that executed mechanical movements, as anticipated by Flemming’s (1882) characterization of the cell spindle as pulling chromosomes apart in mitosis. Of particular importance was research on the cytoskeleton (consisting of actin fibres and microtubules). The term cytoskeleton was invoked in the 1930s and 1940s to denote a rigid structure, and indeed it does help give cells their shape. But it was soon found to be the locus of movement. H. E. Huxley (1969) advanced an account of muscle contraction as resulting from myosin molecules forming bridges that pull on actin filaments. Both actin filaments and microtubules were found to continually extend themselves at one end by incorporating new proteins while removing them from the other, a process characterized as treadmilling (Cleveland 1982). Video observations of organelles moving along microtubules (R. Allen et al. 1982) led to the discovery of kinesins (Vale, Reese, &amp; Sheetz 1985), molecular motors that walk along microtubules carrying cargo. It also focused new attention on dyneins (Paschal &amp; Vallee 1987), previously only known for their roles in cilia and flagella, as carrying cargo in the opposite direction. Recognition of this continual movement of material within cells gave rise to a new metaphor of the cell as a city with bustling traffic (Vale &amp; Milligan 2000).