1. The rate of exosmosis of water was studied in unfertilized Arbacia eggs, in order to bring out possible differences between the kinetics of exosmosis and endosmosis.

2. Exosmosis, like endosmosis, is found to follow the equation

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in which a is the total volume of water that will leave the cell before osmotic equilibrium is attained, x is the volume that has already left the cell at time t, and k is the velocity constant.

3. The velocity constants of the two processes are equal, provided the salt concentration of the medium is the same.

4. The temperature characteristic of exosmosis, as of endomosis, is high.

5. It is concluded that the kinetics of exosmosis and endosmosis of water in these cells are identical, the only difference in the processes being in the direction of the driving force of osmotic pressure.

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