Using unfertilized eggs of Arbacia punctulata as natural osmometers an attempt has been made to account for the course of swelling and shrinking of these cells in anisotonic solutions by means of the laws governing osmosis and diffusion. The method employed has been to compute permeability of the cell to water, as measured by the rate of volume change per unit of cell surface per unit of osmotic pressure outstanding between the cell and its medium.

Permeability to water as here defined and as somewhat differently defined by Northrop is approximately constant during swelling and shrinking, at least for the first several minutes of these processes.

Permeability is found to be independent of the osmotic pressure of the solution in which cells are swelling.

Water is found to leave cells more readily than it enters, that is, permeability is greater during exosmosis than during endosmosis.

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