I4 THE VITAL PROCESSES 



found the very lowest forms of life. In the second, life 

 reaches its greatest development. The body of man, 

 which represents the highest type of life, is recognized as 

 a group of cells. In this group each cell is usually sepa- 

 rate and distinct from the others, but is attached to them, 

 and is held in place by the intercellular material. 



Protoplasm, the Cell Substance. The cell is properly 

 regarded as an organised bit of a peculiar material, called 

 protoplasm. This is a semi-liquid and somewhat granular 

 substance which resembles in appearance the white of a 

 raw egg. Its true nature and composition are unknown, 

 because any attempt to analyze it kills it, and dead proto- 

 plasm is essentially different from living protoplasm. It 

 is known, however, to be a highly complex substance 

 and to undergo chemical change readily. It appears 

 to be the only kind of matter with which life is ever 

 associated, and for this reason protoplasm is called the 

 physical basis of life. Its organization into separate 

 bits, or cells, is necessary to the life activities that take 

 place within it. 



Structure of the Cell. Though all portions of the cell are 

 formed from the protoplasm, this essential substance dif- 

 fers both in structure and in function at different places in 

 the cell. For this reason the cell is looked upon as a 

 complex body having several distinct parts. At or near 

 the center is a clear, rounded body, called the nucleus. 

 This plays some part in the nourishment of the cell and 

 also in the formation of new cells. If it be absent, as is 

 sometimes the case, the cell is short-lived and unable to 

 reproduce itself. The variety of protoplasm contained in 

 the nucleus is called the nncleoplasm. 



Surrounding the nucleus is the main body of the cell, 

 sometimes referred to as the "protoplasm." Since the 



