The viscosity of the hyaline (granule-free) cytoplasm of the immature egg of Spisula (Lamellibranchia) was determined by use of the Einstein equation for Brownian movement. The value obtained was 4.3 centipoises. Thus this protoplasm has a viscosity similar to that of a concentrated protein solution. The viscosity of the entire cytoplasm, granules and all, is much higher. This is due to the great concentration of granular material. Study of the movement of the large germinal vesicle through the cytoplasm under the influence of different centrifugal forces indicates that the viscosity of the entire cytoplasm, granules and all, does not decrease as the shearing force increases. Hence, apparently, the entire cytoplasm behaves like a true Newtonian fluid--at any rate there is no evidence of thixotropy. The idea that protoplasm is typically a gel is not in accord with the evidence presented here. Additional evidence for the fluidity of protoplasm is provided by earlier information about the osmotic behaviour of cells, the formation of ice crystals in protoplasm, the rapid dispersion of aqueous solutions injected into cells, the outflow of protoplasm from cells broken in calcium-free media, and by magnetic studies.

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