THE CELL AND PROTOPLASM. 79 



matter of simple inference. It was argued that 

 since cell origin begins in this way by simple 

 division, and since the same process can be ob- 

 served in the adult, it is reasonable to assume 

 that the same process has continued uninter- 

 ruptedly, and that this is the only method of 

 cell origin. But a final demonstration of this 

 conclusion was not forthcoming for a long time. 

 For many years some biologists continued to 

 believe that cells can have other origin than from 

 pre-existing cells. Year by year has the evidence 

 for such " free cell" origin become less, until the 

 view has been entirely abandoned, and to-day it 

 is everywhere admitted that new cells always 

 arise from old ones by direct descent, and thus 

 every cell in the body of an animal or plant is a 

 direct descendant by division from the original 

 egg cell. 



THE CELL. 



But what is this cell which forms the unit of 

 life, and to which all the fundamental vital pro- 

 perties can be traced 1 We will first glance at 

 the structure of the cell as it was understood by 

 the earlier microscopists. A typical cell is shown 

 in Fig. 9. It will be seen that it consists of 

 three quite distinct parts. There is first the cell 

 wall (cw) which is a limiting membrane of varying 

 thickness and shape. This is in reality life- 

 less material, and is secreted by the rest of the 

 cell. Being thus produced by the other active 

 parts of the cell, we will speak of it as formed 

 material in distinction from the rest, which is 

 active material. Inside this vesicle is contained 



