THE BIOLOGY OF THE CELL SURFACE 



its many properties characteristic of this state of matter. 

 This is not to say that the colloid state is peculiar to organic 

 compounds, such as proteins and carbohydrates; the most 

 thoroughly investigated colloids are those of inorganic 

 substances. It is to be emphasized that the term, colloid, 

 applies to a state or condition only and not to a kind of 

 matter different from crystalloid. Since we know that 

 protoplasm is in the colloid state, it should not astonish 

 us that it behaves as other matter in this state. ^ 



The specific gravity of the total of the cell-contents is 

 usually greater than that of water. But where much fat 

 is present, as in some fish-eggs, the specific gravity of the 

 cell is lower than that of sea-water, as shown by the fact 

 that such eggs are found floating on the surface of the sea. 

 In some eggs the specific gravity increases after fertiliza- 

 tion, a change undoubtedly due to loss of water. 



The refractive index of a cell is a physical property 

 deserving attention. It depends upon the ground-sub- 

 stance and the cytoplasmic inclusions. With the state of 

 aggregation of these latter, the shape and the water-content 

 of the cell, the refractive index of the cell varies. The 

 influence of these factors is shown during the process of 

 cell-division when the cell's refractive index changes with 

 the dispersion of the inclusions, loss of water and change of 

 form of the cell. 



Another physical property of the protoplasmic system 

 much studied in recent years is its viscosity. The signifi- 

 cance attached to such studies has, in my judgment, been 

 grossly exaggerated. Nothing up to now indicates that 

 viscosity has the importance for the state of being alive 

 assigned it by the various investigators who, with different 

 methods, and even with the same method, obtain widely 

 diverging results and announce conflicting opinions. The 



^ See Svedberg, /p2j, and Heilbrunn, 1^28. 



44 



