46 J. MCA. KATER AND R. D. BURROUGHS. 



excellent condition and some cysts were present. By the eleventh 

 day many cysts were found, while in the oilier series this con- 

 diiion was never reached. The cultures were only fair on the 

 twenty-second day and very many cysts were present. 



We now see that the cultures did excellently at any PH from 

 4-5 to 7.5 and that encystment occurred with equal facility at 

 any point within that range. However, in no experimental case 

 was encystment as great as in the controls. 



This experiment haj been followed only to the twenty-second 

 day, but observation was not discontinued at that time. They 

 were examined on several occasions, but it will suffice to give only 

 one of these, that made on the thirty-fourth day. All cultures 

 were rather poor, so far as active forms are concerned and many 

 cysts were present, though not as many as were found in the 

 controls. This data, together with some which will follow, 

 indicates that it was not the actual hydrogen-ion concentration 

 which prevented the experimental cultures from equaling the 

 controls in rapidity of encystment, but the sudden changing of 

 the medium when the Ph was altered. 



These results are quite contradictory to those of Koffman, and 

 are more in harmony with Brand's conclusion that chemical 

 influences are of little importance in producing encystment. 



By-Products of Metabolism and Food Supply. 



These two are taken up together partly because similar methods 

 were used in dealing with them and partly because both factors 

 entered into some of the experiments. 



An attempt was first made to determine the effect of removing 

 the by-products of metabolism by filtering the cultures through a 

 porcelain filter and replacing the medium with fresh hay infusion. 

 Twenty-four cultures were seeded and six of these had their 

 culture medium replaced with fresh hay infusion every day 

 (series Di) six others every second day (series D2), six others 

 every third day (series D$) and the remaining six were kept as 

 controls (series D^). The medium was not replaced in any of 

 the cultures after the tenth day. 



The results of this work are very startling. Whereas in all 

 previous experiments the controls behaved beautifully, encysting 



