CHAPTER III 

 THE CELL AND THE CELL THEORY 



i. MORPHOLOGY AND PHYSIOLOGY OF CELLS 



THE protoplasm contained in the body of an animal is not one 

 continuous mass, but is separated by membranes and other 

 structures into a great number of minute bodies called cells. 

 These cells, though varying greatly in size and shape, all have 

 the same fundamental plan of structure. The simplest animals 

 consist of a single cell, but the bodies of the more complex forms 

 are made up of millions of these tiny bits of protoplasm. On 

 account of its universal presence in animals and plants, the cell 

 has been called the unit of life. It is necessarily of vast im- 

 portance in the solution of biological problems, and has been 

 much studied by investigators during the past forty years. 



In order to have the structure of a typical cell clearly in mind 

 we will now turn our attention to the following description of its 

 parts. Reference to Figure 5 will make the written description 

 clearer. Since the cell contains protoplasm, which acts like a 

 fluid, it naturally tends to have a spherical form. This condition 

 is seldom realized in nature, for various factors, such as unequal 

 growth and pressure from other cells, modify the shape. The 

 fact that the cell shown in the figure has an oblong outline is, 

 therefore, of no significance. 



The contents of many cells includes not only the active proto- 

 plasm, but also various kinds of passive bodies such as fat glob- 

 ules. These are known as metaplasm. The protoplasm of any 

 cell is made up of two principal kinds of substance, the cytoplasm 

 and the karyoplasm. The nucleus contains all of the latter. It 

 is the central body, and is, for various reasons, regarded as the 



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