1. Surface-spread actomyosin, compressed into fibers, shows biological properties of contractility and enzymic activity.

2. In unloaded contractions, wet and dry weight determinations show no appreciable water loss in contraction. The fibers also evince a strong ATP-ase activity.

3. A structural continuity in the fibers by intermolecular linkages of the component actomyosin molecules is established during the formation of the fibers. Evidence includes their visible longitudinal structural organization, the lack of elongation effect of ATP when under tension, and their ability to lift appreciable loads, so that, like muscle, they can transform chemical energy into mechanical work.

4. Up to a limiting critical weight, the fibers perform more work with increasing imposed weight load.

5. Theoretical aspects are discussed, including the possibility that surface-spread protein is involved in the formation of cell structures. Possible explanations for the relative slowness of the fiber contractions are offered.

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