Abstract
A variety of cells possess particles which show movements statistically different from Brownian movements. They are characterized by discontinuous jumps of 2–30 µ at velocities of 0.5–5 µ/sec or more. A detailed analysis of these saltatory, jumplike movements makes it most likely that they are caused by transmission of force to the particles by a fiber system in the cell outside of the particle itself. Work with isolated droplets of cytoplasm from algae demonstrates a set of fibers involved in both cytoplasmic streaming and saltatory motion, suggesting that both phenomena are related to the same force-generating set of fibers. Analysis of a variety of systems in which streaming and/or saltatory movement occurs reveals two types of fiber systems spatially correlated with the movement, microtubules and 50 A microfilaments. The fibers in Nitella (alga) are of the microfilament type. In other systems (melanocyte processes, mitotic apparatus, nerve axons) microtubules occur. A suggestion is made, based on work on cilia, that a microtubule-microfilament complex may be present in those cases in which only microtubules appear to be present, with the microfilament closely associated with or buried in the microtubule wall. If so, then microfilaments, structurally similar to smooth muscle filaments, may be a force-generating element in a very wide variety of saltatory and streaming phenomena.
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