CHAEACTEKISTICS OF ANIMALS 23 



It is commonly known that all living bodies develop from 

 one relatively simple cell. In a perfectly orderly way, nature has 

 provided for the development of groups of cells from the parent 

 cell, to form, first, the tissues, which in turn form the organs, 

 and systems. Eventually the animal body, that highly complex 

 structure composed of countless millions of these microscopic 

 elements, is formed. 



While all cells are of a common descent, some have undergone 

 great modification as to form and function. This specialization 

 is seen in the grouping of cells that have work of a similar nature 

 to perform. For instance, the fat-cells are very passive, and as 

 they perform the function of storing reserve food material they 

 are placed in those parts where least interference will be had 

 with the more active parts. The cells lining the air-passages 

 are very active and their cilia are in continual motion. Other 

 cells, such as certain kinds found in the blood, are still different 

 and possess the power to move from place to place. 



By developing along one definite line, cells lose their primitive 

 power to receive and utilize all kinds of food. Nature, therefore, 

 has provided certain standard forms of food for their sustenance. 

 However, a cell can feed only on that with which it comes in con- 

 tact. Food is supplied to. each cell and waste is removed by the 

 blood and tissue fluids. The exact manner by which this exchange 

 is made possible will be discussed in later chapters. It ' is 

 evident that no cell or group of cells is sufficient unto itself, but 

 relies on the normal interchanges, which appear to be of a chemical 

 nature, that take place with other cells. 



Although a detailed study of cell structure can be accomplished 

 only with the aid of a microscope and is therefore beyond the scope 

 of this book, it is necessary, before considering larger structural 

 parts of the body, to note the general characteristics of typical 

 animal cells (Fig. 2). Each possesses a cell body surrounded by 

 a cell wall, a cell nucleus, a nucleolus, and one or more 

 centrosomes. 



The cell body or protoplasm is the basis of the cell and life proc- 

 esses and will be described fully in the succeeding paragraphs. 

 The cell wall or capsule confines the other parts; it is not so 

 nearly complete in the animal as in the plant; furthermore, 

 in the former it is composed of protein substances, while 

 in the latter it is of carbohydrate material. The cell nucleus or 



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