64 THE STORY OF THE LIVING MACHINE. 



cell. In such a history each cell comes from pre- 

 existing cells and a cytoblastema plays no part. 



It was impossible, however, for Barry or any 

 other person to follow the successive divisions of 

 the egg cell through all the stages to the adult. 

 The divisions can be followed for a short time 

 under the microscope, but the rest must be a mat- 

 ter of simple inference. It was argued that since 

 cell origin begins in this way by simple division, 

 and since the same process can be observed in 

 the adult, it is reasonable to assume that the same 

 process has continued uninterruptedly, and that 

 this is the only method of cell origin. But a final 

 demonstration of this conclusion was not forth- 

 coming 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 en- 

 tirely 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 properties can be traced ? We will first 

 glance at the structure of the cell as it was under- 

 stood 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 

 lifeless 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. 



