16 ACTION OF AUXETICS AND KINETICS 



a genus for the whole group, and to distinguish its 

 members by subdivision into species. 



Amoebae are of minute size, only the larger 

 varieties being visible to the naked eye ; and they 

 are suitable objects for examination with the 

 microscope, as they can be watched continuously 

 while discharging the functions which are char- 

 acteristic of a living cell. The body of each 

 individual amoeba consists of a mass of protoplasm 

 which in some species is regarded as almost naked 

 and in others is undoubtedly invested by a distinct 

 cuticle, the density of which may limit the move- 

 ments of the cell very considerably. Imbedded in 

 the cytoplasm is the nuclear matter or chromatin, 

 which may exhibit an extraordinary diversity of 

 distribution in different circumstances, ranging 

 from a solid-looking mass (nucleus) to a large 

 number of almost imperceptible granules (chro- 

 midia). Under natural conditions amoebae will 

 only live in the presence of living bacteria, and this 

 association has often been regarded as an example 

 of " symbiosis," but quite erroneously, as there is 

 no mutual benefit. The bacteria are utilised as 

 food by the amceba, and it is generally understood 

 that the nutrition of this cell is dependent on the 

 ingestion of solid particles whose chemical com- 

 position is of a protein nature, as amoebae appear to 

 be unable to build up their protoplasm from simpler 

 substances. This characteristic of the amoeba is of 

 importance to these researches, as it will be shown 

 later that the influence of food can be discriminated 

 from the action of auxetics and kinetics in exciting 

 cell-division. 



It is essential for the continued existence of 



