58 



BIOLOGY AND HUMAN LIFE 



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we see, not the protoplasm, but the walls of millions of these 

 cells. In the larger plants and animals the outer layers of cells 

 are usually dead ; that is, the protoplasm is no longer present, 

 only the dead walls remaining. This is true, for instance, of 

 our own skin, of the bark of trees, or of the hide of the horse. 



The microscope 

 enables us to see 

 the forms of many 

 kinds of cells taken 

 from the bodies of 

 plants and animals. 



Cells are different 

 from each other not 

 merely in size but 

 in shape as well. 

 Some cells have 

 thicker walls, some 

 have thinner walls. 

 Some seem to have 

 various kinds of 

 solid bodies float- 

 ing about within 

 the wall ; others 

 have a very few or 

 none of these. Some 

 have smaller and 

 some larger bubbles 





Fig. 29. The one-celled animal, ameba 



The ameba has no definite shape, but moves about, 

 pushing its jellylike mass now in one direction, now 

 in another. After reaching its full growth the nucleus, 

 or kernel, lengthens out and gradually divides into two 

 parts. The rest of the animal's body also lengthens, 

 and the two ends seem to move slowly away from each 

 other until there are two distinct individuals. Each of 

 these is as complete as the other, and both are the same 

 as the mother cell except for size 



of clearer liquid. 



In some plant cells the protoplasm can be seen to move about. 

 This movement, always present in live protoplasm, is easily seen 

 in the cells of certain water plants (Fig. 27). 



46. Nucleus. One special portion of the protoplasm deserves 

 particular notice. Near the center, or at one side, we can gen- 

 erally find a portion that seems to be denser than the rest. This 

 is called the kernel, or nucleus. Since the protoplasm is trans- 

 parent, it may be difficult to distinguish the parts in many 



