60 ENCYSTMENT AND EXCYSTATION OF AMCEBA 



cent, solution of caustic soda for a week ; they 

 will stand severe pressure under a cover-glass, 

 exposure to free hydrogen, oxygen, or ammonia for 

 twenty-four hours, and some will stand 3-per-cent. 

 formalin for a week. They can also be dried over 

 sulphuric acid or phosphoric pentoxide for six weeks, 

 and they can be left on agar slopes for six months 

 without obvious deterioration. 



Effect of alkalinity of the medium on excysta- 

 tion. Early in the work it was suggested to us that 

 variations in the reaction of the medium might 

 play an important part in the processes of 

 encystment and excystation, and to determine this 

 point a solution of caustic soda was used, from the 

 fact that in low degrees of concentration this 

 substance will inhibit bacterial growth. It was 

 thought that, by this means, excystation might be 

 effected in the absence of the products of the 

 growth of living bacteria, and this was attained by 

 the use of a 0*l-per-cent. solution as follows : To 

 each of six test tubes was added 2 cc. of nutrient 

 peptone broth (faintly alkaline). Each of these 

 was rendered alkaline to a different degree by the 

 addition of caustic soda (2*5 per cent.), and the 

 balance was made up to 5 cc. in the following 

 way: 



AMOUNT OF ALKALI IN TUBES 



No. of tube 



1 2 2-98 0-02 O'Ol 



2 2 2-90 010 0-05 



3 2 2-80 0-20 0-10 



4 2 2-50 0-50 0-25 



5 2 2-00 1-00 0-50 



6 2 1-00 2-00 I'OO 



