THE CELL AND PROTOPLASM 



9 



or protoplasm, as it is termed, is a living substance. We 

 know of no case where life exists apart from protoplasm, 

 and for this reason the latter is frequently termed the 

 physical basis of life. 



In addition to the features already described, the proto- 

 plasm of every perfect cell is modified upon the interior to 



FIG. 4. Different types of cells composing the body of the squirrel or other highly 

 developed animal. A, liver-cell; /, food materials; n, nucleus. B, blood-cell. 

 C, nerve-cell with small part of its fiber. D, muscle fiber. E, cells lining the 

 body cavity. F, lining of the windpipe. G, section through the skin. Highly 

 magnified. 



form a well-defined spherical mass known as the nucleus. 

 Other structures are known to occur in the typical cell. 

 Experiment shows that the nucleus and cell protoplasm are 

 absolutely indispensable, whatever their size and shape, and 



