LIVING AND NON-LIVING OBJECTS. 39 



up of certain complex units which we call " cells." These 

 cells are microscopic in size, and while they are not just 

 alike in different organisms nor in different parts of the 

 same organism, they have certain definite parts which 

 are reasonably constant. See Figs. 7 and 8 for some 

 types of cells and for the elements of structure which they 

 show. 



The essential part of the cell is the contents, or proto- 

 plasm. It is usually bounded and protected by a cell wall 

 (Figs. 2 and 3) which is secreted by it. The nucleus (Fig. 

 2 nu.) is a differentiated part of this protoplasm, appa- 

 rently. 



In the lowest animals the whole organism is made up 

 of just one such cell. The highest organisms all begin 

 life as a single cell. Thus we see that the single cell is 

 capable of life, and that the real differences between 

 living and non-living things depend on the cell and its 

 substance. 



52. Protoplasmic Organization. Notwithstanding the 

 minute size of cells, it must not be supposed that the 

 protoplasm which forms it is a simple substance. It is 

 the most complex substance known. Indeed, it is not to 

 be considered as a single substance, but rather as a com- 

 plex mixture of substances, which are continually bring- 

 ing about changes in one another. Some of the different 

 kinds of substances known to be common in protoplasm 

 are water, inorganic salts in solution in the water, 

 starch, sugar, fats, proteid compounds similar to white of 

 egg, ferments, and waste products like carbon dioxide 

 and urea. 



Most of these substances cannot be considered really living 

 materials. The water and starch and urea cannot be 



