138 The Unity of the Organism 



the meshwork proper — indeed, it is by no means certain 

 tliat any adequate ground of distinction exists." ^'"^ 



(c) Three Main Theories of the Structure of Protoplasm 



Wilson then sets forth in general terms the three lead- 

 ing interpretational views of the nature of what is here 

 described ; that is, the well-known reticular or filar theory, 

 the alveolar theory, and the granular theory. It may not 

 be superfluous to state briefly what each of these theories 

 is. The reticular theory interprets what is seen in the 

 protoplasm in its simplest form, as a network of actual fi- 

 bers which branch and anastomose with one another so as 

 to make, according to the familiar comparison, something 

 like the network of a sponge, the spaces within the retic- 

 ulum being filled by the fluid or semi-fluid portions of the 

 protoplasm. This view holds that the granules, supposed 

 by the older observers to be essential constituents of the 

 protoplasm, are the angles where the threads join, the 

 threads seen end-on, and in some cases true granules at- 

 tached to the threads. The alveolar theor}', proposed and 

 defended with great detail of observation and argument by 

 O. Butschli, compares the protoplasm with foam rather 

 than with a sponge. It contends that protoplasm consists 

 of separate, closely crowded minute drops of a liquid alve- 

 olar substance suspended in a continuous interalveolar sub- 

 stance, likewise liquid, but of diff'erent nature. According 

 to this interpretation what are taken by the reticular 

 theory to be fibers are the walls of the alveoli, there being 

 in reality no fibers present. The granular theory holds 

 that granules of various sizes and nature are the fundamen- 

 tal constituents of protoplasm, the fluid and the semi-fluid 

 parts, as also the fibers whenever present, being of sec- 

 ondary significance. 



