THE VITAL UNITS CALLED CELLS 1 95 



inactivity, which would be death. In the same way an 

 engine, or any mechanical contrivance, must be used or it 

 will deteriorate and become unworkable. 



Cells compare favorably with engines, especially chemical 

 ones, in still another respect, namely, the much greater 

 speed at which they perform their duties. Like so many 

 things connected with the cell we understand this quahty 

 but imperfectly. It is known, however, that the rate of 

 chemical reactions is hastened by pecuhar substances 

 termed enzymes, which are of many kinds and are widely 

 distributed in Hving matter. While they accelerate chemical 

 changes thej^ have the property of maintaining their integrity 

 so that they are not lost in the process, but may play their 

 part again and again. Catalysts are widely used in industry. 



ObviousI}^ some internal mechanism for the separation 

 and integration of chemical changes is essential for the cell 

 and the engine. If the contents of a cell are stirred up and 

 mixed together it dies, just as the efficiency of any engine 

 would be destroyed if all parts of it were thrown together into 

 one vast heap. Although such organization undoubtedly 

 exists in the cell, how it is brought about in a space so small 

 that we cannot see it with the naked eye is a mystery. 



A certain amount of localization and separation of chemi- 

 cal reactions is, however, made possible by the elements 

 which are microscopically visible in the cell. Some results 

 of recent investigations point to the conclusion that chemical 

 and physical changes of great variety are prone to occur at 

 surfaces of separation between materials of different char- 

 acter. Referring back to Figure 2, it will be seen that we 

 have to consider in this connection: (i) the surface of the 

 cell itself; (2) the surface of the mitochondria; and (3) 

 the surface of the nucleus. The contents of the nucleus are, 

 in addition, shut off from the surrounding fluid cytoplasm 

 in which it is embedded. It is within the nucleus that the 

 physical basis of inheritance is mainly concentrated. This 

 feature of segregation and protection is very important in 

 the preservation, without continual modification, of heredi- 

 tary characteristics (see Chap. 11). The nucleus is the most 

 acquisitive living element known to us. It "hoards like a 

 raven." This will be briefly considered later. 



