THE LIVING SUBSTANCE. 



63 



two sorts, which are not necessarily separated in a given area. 



(a) They are based on differences in character of the discon- 

 tinuous substances or alveolar inclusions ; or on differences in 

 character of the continuous element along optical and physical 

 paths of the interalveolar substance. 



(b) As to the first class, form, size, and arrangement, also 

 physical differences in kind of inclusions, cause optical differ- 

 entiation enabling one to separate areas from each other. 



(c) Leaving out of count for the moment the finer foam struc- 

 ture, I find areal differentiation which is dependent upon the 

 continuous element for its specific optical character, to be formed 

 by a physical arrangement of this along lines dictated by exist- 

 ing, but variable, foam structure ; with physical differences in its 

 quantity and its appearance, the latter being caused by certain 

 physical and physiological changes which have already been 

 referred to and will be described more in detail in Striation and 

 Contractility. 



Physical variations of the foam structure, and the disposition 

 of interalveolar stuff with its possibilities of difference in rela- 

 tion to any given foam structure ; and these things in potential 

 combination with the chemical variations ; surely offer a mag- 

 nificent plasticity of conditions and causes for evolution to 

 make use of. On this basis arise almost innumerable modifi- 

 cations of rhythmic, or intermittent, optical and physical em- 

 phasis and their associated changes, in form of alveoli, etc. 



[64] With these possibilities even Biitschli's structure offers 

 wide range for areal differentiation ; but when, as frequently 

 happens, this is carried down to the finer foam and its reductions,^ 

 the opportunities become so multiplied as to sate imagination. 



Nor is it found in fact that the living substance has ignored 

 the opportunities. 



[65] In ectosarc is seen the type of substance organization for 

 extension and correlation of physiological function. The con- 

 nection between the two sets of facts is more readily understood 

 when it is seen that in just this type of arrangement of the 

 elements the active powers of the substance have full and 

 perfect physical opportunity for their exercise and emphasis. 



^ See above ; also Contractility and Activities — filose. 



