The behavior of the cell depends to a large extent on the permeability of the outer non-aqueous surface layer of the protoplasm. This layer is immiscible with water but may be quite permeable to it. It seems possible that a reversible increase or decrease in permeability may be due to a corresponding increase or decrease in the water content of the non-aqueous surface layer.

Irreversible increase in permeability need not be due primarily to increase in the water content of the surface layer but may be caused chiefly by changes in the protoplasm on which the surface layer rests. It may include desiccation, precipitation, and other alterations.

An artificial cell is described in which the outer protoplasmic surface layer is represented by a layer of guaiacol on one side of which is a solution of KOH + KCl representing the external medium and on the other side is a solution of CO2 representing the protoplasm. The K+ unites with guaiacol and diffuses across to the artificial protoplasm where its concentration becomes higher than in the external solution. The guaiacol molecule thus acts as a carrier molecule which transports K+ from the external medium across the protoplasmic surface.

The outer part of the protoplasm may contain relatively few potassium ions so that the outwardly directed potential at the outer protoplasmic surface may be small but the inner part of the protoplasm may contain more potassium ions. This may happen when potassium enters in combination with carrier molecules which do not completely dissociate until they reach the vacuole.

Injury and recovery from injury may be studied by measuring the movements of water into and out of the cell.

Metabolism by producing CO2 and other acids may lower the pH and cause local shrinkage of the protoplasm which may lead to protoplasmic motion.

Antagonism between Na+ and Ca++ appears to be due to the fact that in solutions of NaCl the surface layer takes up an excessive amount of water and this may be prevented by the addition of suitable amounts of CaCl2.

In Nitella the outer non-aqueous surface layer may be rendered irreversibly permeable by sharply bending the cell without permanent damage to the inner non-aqueous surface layer surrounding the vacuole.

The formation of contractile vacuoles may be imitated in non-living systems.

An extract of the sperm of the marine worm Nereis which contains a highly surface-active substance can cause the egg to divide. It seems possible that this substance may affect the surface layer of the egg and cause it to take up water. A surface-active substance has been found in all the seminal fluids examined including those of trout, rooster, bull, and man.

Duponol which is highly surface-active causes the protoplasm of Spirogyra to take up water and finally dissolve but it can be restored to the gel state by treatment with Lugol solution (KI + I). The transition from gel to sol and back again can be repeated many times in succession.

The behavior of water in the surface layer of the protoplasm presents important problems which deserve careful examination.

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