H. MUNRO FOX 491 



but that they will not react to a region of greater hydrogen ion con- 

 centration caused by dissolved carbon dioxide. The aggregation 

 then in the center of the liquid beneath a cover-slip must be due to 

 the attractive influence of this central region where the respiratory 

 activity of the organisms has reduced the amount of dissolved oxygen. 



The second question — why does an aggregation become a ring 

 surrounding an ever growing clear area — has now to be answered. 

 Do the Bodo leave the central area {a) because the concentration of 

 dissolved oxygen has fallen here below an optimum value for them 

 or {b) because the hydrogen ion concentration has increased too 

 much? 



If (&) is the cause, the flagellates should leave the central area 

 sooner — that is, they should form a ring sooner — in a preparation 

 which was originally more acid than in one originally less acid. For 

 in the former the critical concentration of H ions which would drive 

 out the flagellates would be arrived at earlier. This test was made. 

 One sample of a culture of Bodo was given a concentration of H ions 

 such that when tested with rosolic acid it gave the same yellow color 

 as did tap water saturated with carbon dioxide. A second sample 

 from the same Bodo culture was given a concentration of H ions 

 showing a pink with rosolic acid. These changes in the H ion con- 

 centrations of the two samples had no effect on the activity of the 

 flagellates as judged under the microscope. If the cause of the 

 formation and spreading of the band is the accumulation of carbonic 

 acid at the center of the liquid, the occurrence must take place sooner 

 in a preparation made from the first sample than in one made from 

 the second. The experiment showed, however, that the band was 

 formed and spread simultaneously in the two preparations. 



It must be mentioned here that in this and all other experiments 

 when the times of aggregation or band formation were to be compared 

 in two preparations, the cover-glasses were supported by short pieces 

 of thin glass rod cemented to their corners, not by wax feet. By 

 using pieces of glass having the same thickness it was ensured that 

 the cover-glasses were at the same height above the slides. This 

 is necessary since the height of the cover-glass influences the rate of 

 aggregation and the distance of the equilibrium position of the band 

 from the edge. 



