TYPE OF AMCEBA 17 



living cells that reproduction shall take place at 

 some time or other, and in the amoeba this phe- 

 nomenon presents a large variety of methods which 

 in some instances appear to depend on the presence 

 of particular chemical agents in the environment. 



Lastly, amoebae are able to protect themselves 

 from harmful conditions of their surroundings by 

 forming a resistant envelope round their delicate 

 contents, the process being termed " encystment," 

 and the bodies so formed being called " cysts." 



Under the influence of certain altered conditions 

 the amoebae are enabled to escape from the cysts, 

 and this phenomenon is described as " excystation." 



Type of Amoeba employed. The particular 

 amoeba on which the following experiments have 

 been carried out was found growing in a sodium 

 citrate and sodium chloride solution (3 per cent, 

 and 1 per cent, respectively) which had been left 

 lying exposed to the air of the laboratory for some 

 months. At the outset it was found difficult to 

 classify it with any of the better-known species of 

 amoeba ; at any rate it was difficult to do so upon 

 the characters of its body-form and movements. 

 Its presence also, and active multiplication in a 

 mixture of such a high degree of saline concentration 

 suggested the possibility that, even if it was not 

 a new species, a modification of type had in some 

 way taken place. As subsequent results have 

 shown that the appearance of amoebae, on which 

 their classification is to some extent based, is sub- 

 ject to the environment to which they are exposed, 

 a differentiation into species based on morphology 

 alone is of little real importance. In our opinion it 

 is only by gross differences in habits, life-cycle, and 

 iv 2 



