20 Causes and Course of Organic Evolution 



sufficiently swollen or dissolved in water, alcohol, etc., present 

 marked resemblances to gelatine, white of egg, blood clot, 

 and other organic bodies of colloid nature. The differences 

 between the crystalloid and the colloid groups are specially 

 interesting biologically. For, while each ultramicroscopic 

 molecule that makes up a crystalloid body shows regular and 

 neat facets that fit into neighbor facets of other units to build 

 up a solid relatively stable whole or compound crystal, in 

 colloid bodies the primary molecules are surrounded by a 

 jacket of liquid, and the groups or compound molecules of 

 these are so surrounded and separated by liquid that they 

 fail to fit closely together. 



Each group or compound molecule then — that by Nageli 

 and others has been called a micella — is enveloped by a liquid 

 film, while crystalline bodies, formed by direct apposition 

 of simple molecules, are devoid of enveloping water. The 

 greater irregularity and plasticity of the former also give to 

 such two important qualities: (a) that of readily absorbing 

 and holding water or other liquid in the areas between the 

 complex molecules, and (b) that of permitting a labile motion 

 of the micellse between each other. It thus follows that col- 

 loids are more liable to alteration and to undergo decomposition 

 changes than crystalloids. In other words they are more 

 readily affected by, and respond to, environal action. 



