CELLS AND THEIK ASSOCIATION 39 



are which exceed this scale. The name protoplasm is 

 given to the characteristic material of which cells are 

 composed. In most cases there can be demonstrated 

 in a cell an internal mass which seems denser than the 

 rest. This is called the nucleus. We are in the habit 

 of speaking of the cell as a small mass of living matter 

 but we are compelled to admit when questioned that we 

 do not know what proportion of the whole is truly alive. 

 This is a stimulating but rather hopeless line of inquiry. 

 When cells from different parts of the body are com- 

 pared we can recognize that some show special adapta- 



Fio. 1. To contrast the empty "cells" of a dry, woody tissue, en- 

 closed by substantial walls, with the "cells" of a soft, animal tissue 

 which are separate parcels of living matter. They are related to the 

 other somewhat as casts to their moulds. 



tion to particular uses while others are obviously of a 

 more primitive type. Qells that can be regarded as 

 primitive suggest to one that the standard form is the 

 sphere. Yet this form is seldom perfectly realized, 

 chiefly because cells which are pressed together naturally 

 come to have flattened surfaces at places of contact. 

 Thus they are like grapes that have been packed too 

 tightly. We ought to think of cells as of a very soft 

 consistency. The structural strength of the body as a 

 whole is gained through the development in it of deposits 

 and fibers which are not cellular but intercellular in 

 nature. 



Tissues. Before cells were recognized as characteristic 

 units, into which living matter can be resolved, anato- 



