4 THE CELL 



The process whereby the cell breaks up these complex com- 

 pounds formed by anabolism and discharges them from its 

 substance, is called katabolism, or destructive metabolism. A 

 good example of anabolism is that by which the vegetable cells 

 take such substances as carbon dioxide, water and inorganic 

 salts and prepare food-material for the nutritive and katabolic 

 processes in animals. 



(2) Growth is the natural sequence of the nutritive changes 

 effected by metabolism and may be unrestricted and equal in 

 all directions. However, this is not usually the case as is shown 

 by the fact that cells are so intimately associated with other 

 structural elements as to influence and modify their growth. 

 These result in unequal growth, to which the specialization of 

 cells is due. Examples of the unequal growth of cells are 

 shown in the columnar cells of epithelium, the neurones of 

 nervous tissue, the fibers of muscle tissue, etc. 



(3) Reproduction may be regarded as the culmination of 

 the activities of the cell, for by this process the cell loses its 

 individuality and continues its life in that of its offsprings. 

 There are two methods by which cells may reproduce them- 

 selves, (i) by direct cell division or amitosis and (2) by indirect 

 cell division or mitosis. 



(4) Irritability is that property of cells whereby they are 

 enabled to respond to stimuli, i. ., to change their form and 

 shape in response to these stimuli. The various stimuli which 

 affect cells may be mechanical, thermal, nervous, chemical or 

 electrical. 



Cell Division. (i) Direct cell division, or amitosis, is the 

 simplest form of cell division. In this form of cell division the 

 nucleus and protoplasm constrict in the middle until two new 

 cells are formed. This form of cell division does not occur in 

 the higher animals except as a secondary process. 



(2) In the higher animals cell division takes place chiefly 

 by the indirect or mitotic method. This may be described 



