1. Osmotic equilibrium and kinetics of osmosis of living cells (unfertilized eggs of Arbacia punctulata) have been studied by a diffraction method. This method consists of illuminating a suspension of cells by parallel monochromatic light and measuring, by means of telescope and scale, the angular dimensions of the resulting diffraction pattern from which the average volume of the cells may be computed.

The method is far less laborious and possesses several advantages over direct measurement of individual cells. The average size of a large number of cells is obtained from a single measurement of the diffraction pattern and thus individual variability is averaged out. The observations can be made at intervals of a few seconds, permitting changes in volume to be followed satisfactorily. During the measurements the cells are in suspension and are constantly stirred.

2. Volumes of cells in equilibrium with solutions of different osmotic pressure have been determined. In agreement with our previous experiments, based upon direct microscope measurements, we have confirmed the applicability of the law of Boyle-van't Hoff to these cells; that is to say, the product of volume and pressure has been found to be approximately constant if allowance be made for the volume of osmotically inactive material of the cell contents.

The volume of osmotically inactive material was found to be, on the average, 12 per cent of the initial cell volume; in eggs from different animals this value ranged from 6 to 20 per cent.

3. Permeability to water of the Arbacia egg has been found to average, at 22°C., 0.106 cubic micra of water per square micron of cell surface, per minute, per atmosphere of difference in osmotic pressure.

4. Permeability to ethylene glycol has been found to average, at 24°C., 4.0 x 10–15 mols, per square micron of cell surface, per minute, for a concentration difference of 1 mol per liter. This is in agreement with the values reported by Stewart and Jacobs.

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