THE BIOLOGICAL UNIT 31 



(i) a central, sac-like body, the nucleus; and (2) 

 the larger mass surrounding the nucleus, the cyto- 

 plasm (Fig. 4). 



The number of cells in an animal or a plant is 

 no measure of their relative rank in the scale of 

 life, as the following examples indicate: The ex- 

 tinct mastodon contained thousands more cells 

 than man; the huge Californian redwood trees 

 have more cells than any other living thing, yet 

 they differ only in size, not in function, from our 

 common trees. These facts are made more sig- 

 nificant as one studies the varied appearances of 

 cells as shown in figures 5-1 1. It is not the num- 

 ber of cells in an organism that determines its 

 rank, but the quality of the vital processes that 

 take place within them. 



It is instructive to reflect on the relations of the 

 multitude of cells in the body of man. Are there 

 any classes? Are all equal in importance? Are 

 there some which grow weary and old with service 

 and must forever hide under the organization of 

 the larger unit of man? 



In so far as the higher animals are considered, 

 man is conceded to be the most specialized and 

 highly organized of all. The biological unit in 

 man is differentiated into such tissues as muscles, 

 glands, bones, skin, nerves and blood. Bone cells 

 consist of enormously thickened cell walls with the 

 vital, living protoplasm reduced to a minimum. 



