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EXPERIMENTAL GENERAL SCIENCE 



they can readily be seen with a compound microscope and 

 some of the larger ones may be distinguished with a good 

 lens. A typical cell consists of a rather dense portion, called 

 the nucleus, in which the life processes center, and a more fluid 

 portion, the cytoplasm, surrounding it. In the cells of plants 

 especially, there are one or more cavities, called vacuoles, in 

 which the cell sap is found. Plant cells are usually surrounded 

 by a thin membrane called a cell wall, which is built up by 

 the protoplast, but animal cells seldom have these cell walls. 



Fia. 79. Cells and their contents. A and B, with red and yellow chromo- 

 plasts; C with nucleus and green chloroplasts. (A, after Strasburger; B, after 

 Frank; C, after Stevens.) 



The substances that cells build up are sometimes more con- 

 spicuous than the cells themselves, especially in the harder 

 parts of animal bodies. The simplest animals and plants 

 consist of single cells and, though so small, have all the essen- 

 tial capacities of living things. In more complex forms, 

 there may be uncounted millions of cells, often varying greatly 

 in shape. When similar cells are arranged in groups, they are 

 called tissues. Pith, wood, cork, blood, muscle, and bone are 

 tissues. The tissues are usually combined to form organs, 



