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 
Fic. 4.—Different types of cells composing the body of the squirrel or other highly 
developed animal. A, liver-cell; 7, 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 
