This paper continues the discussion of an immersion method of refractometry of living cells, the basic principles of which were given in Part I (Barer and Joseph, 1954). A suitable immersion medium should be non-toxic, must not penetrate the cell, and must be in osmotic equilibrium with it so that no change in volume (and hence in concentration) occurs. These requirements are best met by solutions of substances of high molecular weight. The effects observed when cells become freely permeable to such substances are described. An account of tests with various immersion media is given. The main substances tried have been peptone, proteose, protein hydrolysate, dextran, polyvinyl alcohol, polyvinylpyrrolidone, acacia gum, egg albumin, bovine gamma globulins, carboxyhaemoglobin, and bovine plasma albumin. Of these, bovine plasma albumin has proved tobe most generally useful though acacia gum may be a good inexpensive substitute for some cells, particularly fungi

The osmotic properties of the immersion medium are most important and must be carefully controlled if true determinations of solid concentration are to be made. Some of the many difficulties in defining an isotonic physiological medium are reviewed. It is suggested that the best practical definition is that such a medium should be innocuous and should not change the cell volume. A method of adjusting the salt content of the medium, based on this definition, is described, and the possibility that some cells may exhibit a degree of osmotic regulation is discussed. Finally, evidence is presented to demonstrate the harmlessness of the isotonic protein medium to many types of cells-Photomicrographs of cell division over a periodof 48 hours in a protein medium are shown.

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