Chapter II 



THE CELL. ITS NATURE, FUNCTION OF ITS PARTS, AND 



ITS SOURCE OF ENERGY 



Structure of the Cell. Living objects are not merely masses 

 of a generalized substance, protoplasm. The protoplasm is organ- 

 ized into various types and into structural and functional units 

 called CELLS (Fig. 4). The term is somewhat of a misnomer, since 

 the units are not empty, nor fluid-filled cavities, but are compact 

 masses of protoplasm, each mass usually surrounded by a limiting 

 MEMBRANE. Within this limiting boundary is to be found proto- 

 plasm of various types. Thus, each cell contains at or near its centre 

 a highly specialized mass of protoplasm called the nucleus. The 

 nucleus, except when it is undergoing division, is bounded by a 

 thin membrane of a still different type of protoplasm, the nuclear 

 MEMBRANE. The greater volume of the cell is occupied by a type 

 of protoplasm called cytoplasm. In the cytoplasm are to be found 

 specialized masses, more dense than their surroundings, called 

 PLASTiDS, of which there appear to be many sorts. Adjacent to the 

 nuclear membrane, in animal cells at least, is to be found an impor- 

 tant, though small, body, the centrosome. In the vicinity of the 

 centrosome the cytoplasm is somewhat different in appearance from 

 the remainder; this region is known as the centrosphere. Rod-like 

 structures are frequently found scattered throughout the cell out- 

 side the nucleus. These are named mitochondrl\. They appear to 

 be of regular occurrence, but their significance is still a matter of 

 dispute. The outer boundary of the cytoplasm is itself a distinct 



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