Many investigations have been made of the changes in weight of frog muscles after immersion in different solutions. The quantitative interpretation of the results has always been difficult because of the impossibility of knowing whether the observed changes in weight of the whole muscle were due to changes in the fibers themselves or to changes in the extracellular fluid. Evidence has recently accumulated (Fenn, Cobb and Marsh, '34) to show that the volume of the tissue spaces can be estimated from the chloride content of the muscle, since all the chloride is contained in the extracellular solution. The volume of this extracellular fluid, or the chloride space, is calculated from the formula
Chloride percentage in muscle Chloride percentage in solution
Making use of this method, therefore, it seemed of interest to repeat familiar experiments on weight changes of frog muscles in hypotonic and hypertonic solutions in order to discover whether the volume of the fiber water (i.e., the volume of the fiber minus the dry matter) would vary inversely as the osmolar concentration of the solution. If the fiber behaves as a simple osmometer, the product, V C , of volume and concentration should be a constant. If, however, a certain fraction of the fiber water is osmotically inactive, either because it is 'bound' in some fashion, or because it is not enclosed in a semipermeable membrane (which completely 93