i6 



THE VITAL PROCESSES 



How the Cells enable the Body to Grow. Every cell is 

 able to take new material into itself and to add this to the 

 protoplasm. This tends to increase the amount of the 

 protoplasm, thereby causing the cells to increase in size. 

 A general increase in the size of the cells has the effect of 

 increasing the size of the entire body, and this is one way 

 by which they cause it to grow. There is, however, a 

 fixed limit, varying with different cells, to the size which 

 they attain, and this is quite low. (The largest cells are 

 scarcely visible to the naked eye.) Any marked increase 

 in the size of the body must, therefore, be brought about 

 by other means. Such a means is found in the formation 

 of new cells, or cell reproduction. The new cells are al- 

 ways formed by and from the old cells, the essential process 

 being known as cell-division. 



Cell-Division. By dividing, a single cell will, on attain- 

 ing its growth, separate into two or more new cells. The 



FIG. 5. Steps in cell-division (after Wilson). Note that the process 

 begins with the division of the attraction sphere, then involves the nucleus, 

 and finally separates the main body. 



process is quite complex and is imperfectly understood. 

 It is known, however, that the act of separation is pre- 

 ceded by a series of changes in which the attraction sphere 



