PHYSIOLOGICAL 393 



brane behaves like a semi-permeable membrane of this type, as was 

 shown by de Vries in his experiments on plasmolysis. The water 

 within the cell is always a solution of various salts and soluble organic 

 compounds; suppose then that a cell is immersed in pure water or 

 in a weaker solution (a solution with fewer dissolved molecules in a 

 given volume is said to be hypotonic), a change will take place; 

 water will pass from the weak solution outside to dilute the stronger 

 solution inside. The physical reason for this cannot be discussed 

 here; it is enough to say that two solutions have so strong a 

 tendency to equalise any difference in strength between them, 

 either by diffusion of the dissolved molecules, or, if this is not 

 possible, by movement of the water from the weaker to the stronger, 

 that water will enter the cell from the weaker liquid outside till 

 there is a considerable pressure {osmotic pressure) within. This 

 pressure or tiurgidity is of great service in maintaining plant-cells 

 in a more or less rigid state. Their cell walls are strong enough to 

 withstand the pressure, but a red blood corpuscle immersed in a 

 weak solution will take up water till the cell membrane bursts and 

 the contents (the red pigment haemoglobin) will be scattered into 

 the water; this is known as "laking" the blood. 



If, on the other hand, a cell is immersed in a strong {hypertonic) 

 solution, water will pass from the cell to dilute the external fluid, 

 and the cell will be seen to shrink ; this is the phenomenon of plas- 

 molysis studied by de Vries. Cells, or plants and animals generally, 

 differ in their power to survive such changes of the osmotic pro- 

 perties of their environments. In general, it is found that the body 

 fluids of marine Invertebrates have much the same osmotic pressure 

 as sea water; they are isotonic. It is a remarkable fact that the salts 

 present in these fluids, and indeed also in the blood of vertebrates, 

 show much the same relative proportions as in sea water; but 

 within the cells there is typically much more potassium and much 

 less sodium than in sea water. This fact, that cells take up some salts 

 in preference to others, is a clear indication that the picture of the 

 surface film of protoplasm as a semi-permeable membrane must not 

 be looked on too simply. 



About the beginning of this century, Overton made a series of 

 experiments on the permeability of the cell surface to various sub- 

 stances, and found that those which entered the cell most easily 

 were organic compounds soluble not so much in water, as in the 

 "fat -solvents" such as benzene, ether, and the fats themselves. Most 

 of them had a narcotic influence on the cell. He therefore supposed 

 that the cell membrane was composed of fat-like substances, 

 lipoids; and this view has always had much to commend it. Never- 

 theless it is at the best too simple a theory to explain all the known 

 facts. 



Before considering later theories of the nature of the cell surface 



