THE CELL 93 



many cases, there are others, even when we know that the most mul- 

 tifarious chemical reactions are being carried on, in which nothing of the 

 mechanism is recognizable to our eyes." 



In plant cells where the cell wall is quite thick, and in some of the 

 animal cells, this cell wall is made up of cellulose, a substance quite 

 clearly related to the starches, although there are other substances, such 

 as lignin or silica, often associated with it, while in the cell walls of 

 animals there is a nitrogen containing substance, such as chitin, keratin, 

 and gelatin. 



Where there is no distinct cell wall, there may be a cuticle, or 

 pellicle, covering the entire cell. This may be considered a lifeless secre- 

 tion, just the same as is the cell wall produced by some of the vital 

 activities of the cell itself. The vacuoles are little open spaces or vesi- 

 cles of liquid enclosed within the protoplasm. They may be persistent 

 or merely temporary. In protozoa, vacuoles are quite common. If they 

 enclose food particles, they are called food vacuoles. They may, by con- 

 tracting suddenly, eject their contents and serve thus as. excretory or- 

 gans. As these vacuoles which eject their contents usually are formed 

 again in the same place, they are called pulsating or contractile vacuoles. 



