4 THE CELL 



The second process, that by which the cell breaks up these 

 complex compounds 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 epithe- 

 lium, 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. e., 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 



