48 



WHAT IS BEING ALIVE? 



work form organs. Such an organ is a leaf, made of supporting 

 cells, green cells, spongy cells, etc. : or the human arm, with its 

 bony supporting tissue, its nerves and muscles, its blood vessels 

 and connective tissue. 



How cells form others. Cells can grow only to a certain size. 

 When this limit is reached, thccell splits, forming two ceUs. In 

 this process, which is of very great importance in the growth of 

 both plants and animals, the nucleus elongates and divides : the 

 halves separate and go to opposite ends of the ceU. Then the rest 

 of the protoplasm divides equally and two cells are formed, each 



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 Cell & 



Plant and animal cells multiply by division. 



containing a nucleus. Each cell will have exactly the same char- 

 acteristics possessed by the original cell. This process is known as 

 direct cell division. Usually a more complicated process of division 

 known as mitosis occurs in most cells. See diagram on page -i9. 



The chromosomes and their functions. If we now examine a 

 specially prepared and stained cell,, for example,, the egg cell of a 

 worm or a frog, we shall find that the nucleus, when stained with 

 certain dyes, shows numerous small deeply stained bodies within 

 it. These structures are called chromosomes (kro'mo-somz j 

 Gr. chroma, color; soma, body), or color-bearing bodies. The 

 number of these chromosomes in each body cell of a given kind of 

 plant or animal is always the same. For example, forty-eight are 

 found in man, four in a certain worm, and eight in one kind of lily. 

 In plants and animals there are two distinct kinds of cells, one 

 group called the somatic or body cells, which form the bulk of the 

 body, and the sex cells which pass on the heredity qualities to the 

 next generation. The sex cells are able to do this by means of 



