GROWTH, AGE, AND DEATH 301 



plasm beyond that revealed by the highest powers of the microscope 

 and by such other observations as are available, so we do not know 

 the exact nature of the processes that result in the irreversible cessa- 

 tion of protoplasmic activity, that is, death. But as a matter of fact, 

 much that is known concerning the vital processes has been gained 

 by studies on the processes of disorganization preceding death. The 

 fact that protoplasm is a colloid and hence is subject to the laws 

 governing colloidal behavior suggests the probabilities that underlie 

 death of the cell. By definition a colloid consists of matter suspended 

 in a finely divided state. Many sorts of agents cause the finely di- 

 vided particles to clump together, a process known as coagulation. 

 The coagulation of a colloid represents its disorganization, its 

 change from one type of structure to another. In some colloids 

 coagulation is irreversible; for example, the coagulation of egg 

 white by heat or by alcohol. Conditions similar to this type of 

 coagulation are observed when protoplasm dies, and some biolo- 

 gists have expressed the view that death is primarily an irreversible 

 coagulation of protoplasm. 



The physical changes that mark the irreversible disorganization 

 of dying protoplasm are accompanied by chemical changes. But 

 some of the chemical processes of metabolism are not extinguished 

 by the destruction of the living structure of the cell. If a cell is 

 ground up with sand or otherwise crushed and mechanically dis- 

 integrated, the remaining formless mass continues to consume 

 oxygen for a time. From such preparations it is possible to extract 

 enzymes that remain active. But enzymes, the organic catalysts, are 

 not alive; their activity merely plays a part in the total metabolism 

 of the protoplasm. Again we are impressed with the evidence that 

 life is a consequence of organization, not of specific chemical 

 processes nor specific substances. 



Let us return for a moment to the subject matter of the first 

 chapter. In the light of the facts outlined in the present discussion 

 one may add still another characteristic of life to those listed in 



