The osmotic behavior of cells of solid tissues of mammals has apparently not been studied. Free cells, like erythrocytes and other blood cells, have received considerable attention in this connection, but similar studies do not appear to have been reported for cells of parenchymatous tissues. In much of the work which has been done on water exchange in animal cells, eggs of various aquatic invertebrates were used as experimental material. Conclusions drawn from the osmotic behavior of such cells are not entirely applicable to mammalian cells, since the normal environmental fluid in one case contains protein and, in the other case, does not contain protein. Nor are results obtained with mammalian erythrocytes strictly applicable to cells of solid tissues for, as Lucké and McCutcheon (1932) stated in their review of the osmotic properties of the living cell, the mature mammalian erythrocyte “is a very unique kind of cell, if indeed it may be called a living cell at all, and because of its peculiar structure and function, its osmotic properties are perhaps more influenced by slight environmental changes than is the case with any other kind of cell. … Failure on the part of numerous investigators in the field of osmotic phenomena to take into account the peculiar properties of erythrocytes has led to an astonishing diversity of results, and, indeed, to the denial by some … that the cell is an osmotic system.” Some work has been done on fluid exchange in isolated tissues or parts of tissues. But, as these authors pointed out, “tissues contain intercellular structures which apparently possess properties different from those of the cells. In other words, the osmotic properties of tissues cannot justifiably be assumed to be identical with those of their component cells.”