THE ORGANIZATION OF THE INDIVIDUAL 15 



tively small changes in temperature or other agents; and their liability to 

 become irreversibly solid ("set") or fluid when subjected to greater 

 changes or when acted upon by chemical agents is another striking fea- 

 ture. These changes are the result of complex and delicate relationships 

 between the discontinuous particles and the continuous medium about 

 them. At one temperature, for example, the solid substance may be 

 dispersed and the colloid be a fluid, while at a lower temperature the 

 solid may be the continuous phase, and the colloid be a gel. In protoplasm 

 the situation is enormously more complicated because of the many and 

 changing kinds of discontinuous particles involved. 



Many of the other properties of colloids are caused by the tremendous 

 amounts of surface provided by the billions of discontinuous particles, 

 for these surfaces are the site of peculiar properties of adsorption and 

 electric charge. A great deal of the chemical activity within the cell is also 

 concentrated along these boundaries and may occur at rates which would 

 be impossible except for the immense area of contact between the reacting 

 substances which the colloidal state provides. 



The modern worker is still unable to analyze, explain, or synthetically 

 duplicate metabolism, reproduction, and irritability, but he finds no 

 evidence that these result from any nonphysical property of protoplasm. 

 The problems of protoplasmic structure and function appear to differ 

 from other problems of matter only in their unique complexity. 



THE STRUCTURE OF THE CELL 



The simplest description of a cell would be "a small mass of protoplasm 

 differentiated into nucleus and cytoplasm." 1 The actual size of this small 

 mass varies greatly between different kinds of cells. If we disregard such 

 specialized exceptions as yolk-gorged eggs, the maximum size is well 

 below a cubic millimeter (about 1/16,000 cubic inch), and the minimum 

 less than one-thousandth of this. Such a size range extends from cells 

 that are easily seen with the unaided eye to ones that require the higher 

 powers of a microscope to be at all visible. 



The shapes of these masses are almost as varied as their sizes. A bit of 

 protoplasm free from stress or pressure tends to take a spherical form, 

 and not a few cells have this shape. The great majority of cells, however, 

 have various nonspherical shapes. The cells in tissues subject to strains^ 

 stresses, and pressures of various kinds may be cubical, cylindrical, 

 flattened, keystonelike, or spindle-shaped, as shown in Fig. 2.3. Most 

 nerve cells have long, slender, branched processes that extend far out 



1 The lowly bacteria and blue-green algae do not show differentiation into nucleus 

 and cytoplasm; in them the entire cell seems to function somewhat like a nucleus. 

 Various other special exceptions to the conditions here described as typical are to be 

 found in different groups of organisms. 



