EXCYSTATION 71 



on this medium the amoebae thrived and multiplied 

 all over the tube. It is thus apparent from these 

 experiments that the amoeba requires, for its satisfac- 

 tory cultivation without living bacteria, the presence 

 of some soluble substances which are formed by 

 bacteria and which are destroyed by boiling but 

 not by chloroform (vide infra}. Under natural 

 conditions these substances are provided by the 

 growth of the living bacteria. It was suggested by 

 Liston that some products of living bacteria are 

 essential for the continued reproduction of amoebae, 

 and we believe that these substances are of an 

 extracellular nature and can be applied to the 

 amoebae in the form of an aqueous solution which 

 contains the products of bacterial growth. 



Pure cultures of amoeba. We are in some doubt 

 as to what is generally understood by this expres- 

 sion. We have been able to grow amcebaa in the 

 absence of living bacteria, and it appears to us that 

 a definition of the term " pure culture " should be 

 satisfied by these conditions. The dead bacteria, 

 killed by boiling, which we have employed, have no 

 more significance to us than if they were particles, 

 regarded merely as dead food. At the same time 

 the amoeba does require more than this before it 

 will multiply continuously, namely, some substances 

 produced by bacteria which are destroyed by boil- 

 ing, but not by chloroform. It also requires 

 auxetics or kinetics (preferably both of these) to 

 enable these substances (which appear to be of the 

 nature of ferments) to act on the cell. If, however, 

 by the term " pure culture " is meant one in which 

 the amoeba is able to multiply on a medium which 

 has been boiled and contains no solid particles,. 



