Seven experiments are described which permit estimation of the "solvent water" or the "osmotically active water" of the dissected fiber from the giant barnacle, Balanus nubilus. Each of the first four experiments includes the measurement of a free ion activity in the myoplasm by means of a Na+, K+, or Cl- ion-specific microelectrode. The fifth experiment makes use of a membrane potential vs. [K]o curve. The last two experiments measured fiber water and fiber volume as bath osmolarity was changed. The seven independent estimations of solvent water ranged from 0.64 to 0.72 of fiber water with a mean of 0.68. Since the extracellular space of single fibers was about 7% of fiber water, it was concluded that 25% of analyzable water was not acting as solvent for the osmotically active solutes in the myoplasm.

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