CHAPTER II 



THE COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 

 i. THE CELL 



73. Definition. The cell may be defined as the biological 

 unit of all life. It is the simplest form in which living matter 

 can exist. It might be regarded as bearing somewhat the same 

 relation to the animal or plant that the atom does to a complex 

 organic molecule such as that of one of the proteins for example. 

 It is seen in its simplest form in unicellular organisms (protozoa) 

 in which all the functions of life are performed by a single cell. 

 As we ascend in the scale of organization a number of cells are 

 united to form one individual, the various vital functions being 

 to a greater or less extent distributed among different cells or 

 cell groups. In the higher organisms the cells are numbered 

 by myriads, while the physiological division of labor and the 

 corresponding differentiation of form reach an extreme. The 

 organization of such an individual has been likened to that of 

 a state or nation, in which the functions of the single citizen 

 are highly specialized. A few of the diverse forms of animal 

 cells are represented in Fig. i. 



74. Structure of cells. The typical cell consists of the 

 cell body, or cytoplasm, within which is the nucleus. The 

 peripheral portion of the cytoplasm is often somewhat more 

 compact than the remainder and serves to separate the cell 

 from its surroundings. Sometimes a distinct membrane, or 

 cell wall, is developed, especially in plants, although this is not 

 a necessary part of the cell. The name protoplasm is often 

 applied to the entire active part of the cell, i.e., to cytoplasm plus 

 nucleus. All forms of life, vegetable as well as animal, are in- 

 dissolubly associated with and manifested through the activities 

 of protoplasm, which was called by Huxley the physical basis 

 of life. It should be understood, however, that the word pro- 

 toplasm is not a chemical but a biological term. It is a struc- 



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