CHAPTER I 

 How the Animal Body is Fonned 



The cell is the unit of growth. It is so with all 

 forms of life — plant or animal, insect or bacterium. 

 In the beginning the start is with a single cell, an 

 ^SSy if you please. After fertilization has taken 

 place, this single cell enlarges or grows. Many 

 changes now occur, all rather rapidly, until the cell 

 walls become too small, when it breaks apart and 

 forms two cells just like the first used to be. This 

 is known as cell division. As growth increases, the 

 number of cells increases also — until in the end 

 there are millions. 



Nature of the Cell. — The cell is very small. In 

 most cases it cannot be seen with the naked eye. 

 The microscope is necessary for a study of the 

 parts, the nature and the character of the cell. 



In the first place the cell is a kind of inclosed 

 sac, in which are found the elements of growth 

 and life. Surrounding the cell is a thin wall known 

 as the cell membrane. In plants this cell wall is 

 composed of cellulose, a woody substance, which 

 is thin and tender in green and growing plants, but 

 hard and woody when the plant is mature. 



Within the limits of the cell is the protoplasm, 

 the chief constituent of the cell ; locked up in this 

 protoplasm is life, the vital processes that have to 

 do with growth, development, individual existence. 



Embedded within the protoplasm is another part 

 known as the nucleus and recognized under the 

 microscope by its density. Around the nucleus is 



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