ON THE STRUCTURE OF CELLS 139 



living matter ? " And the answer is that, " we have 

 but one criterion to assist us to a decision, and that is 

 metabolism ; only when there is metabolism there is 

 life. This is the A B C of physiology. Thus the 

 starch granule in the plant cell, the glycogen particle 

 in the animal cell, so long as they remain without 

 chemical change, as reserve material in the proto- 

 lasm, are not living. They only become living 

 when, decomposed and dissolved, they take part in 

 metabolism .... Given unchanging outward con- 

 ditions, the form depends wholly on the unchanging 

 character and direction of the metabolism. In the 

 fountain or gas-jet, although the particles are in a 

 constant movement, not one of them retaining the 

 same position for a moment, the entire jet retains 

 its characteristic form as long as it is subject to the 

 same conditions. A cell, in spite of its active 

 metabolism, appears to us to be in repose. It gives 

 us the impression of immovability, just as a gas or 

 water jet conveys the same impression, although each 

 particle is in rapid and incessant movement." Living 

 and dead protoplasm may be illustrated by the gases 

 or materials in a flame, they may be said to represent 

 two different states of matter, even though the 

 chemical composition be the same. 



We shall see that even solids such as phos- 

 phorescent bodies can thus exist in two different 

 states, although the ultimate chemical composition 

 is the same in the two cases. Metabolism is a 

 process of building up and breaking down of 

 molecules. It is difl&cult to see how this process 



