510 SPONTANEOUS AGGREGATION OF FLAGELLATES 



lessened oxygen content arise towards the center of the liquid beneath 

 the cover-glass, owing to the oxygen consumed by the flagellates in 

 respiration not being replaced here by the solution of atmospheric 

 oxygen, as it is along the edges of the liquid. The flagellates, however, 

 are insensitive to the attraction of regions of lessened oxygen con- 

 centration when the oxygen concentration throughout the liquid is 

 above a certain value. Therefore, for the aggregations to form, 

 either the initial concentration of dissolved oxygen must be below this 

 limiting value, or an interval of time must first elapse after the making 

 of the preparation until the respiration of the organisms has reduced 

 the oxygen concentration throughout the liquid down to this limiting 

 value. The aggregations will then form because the flagellates have 

 become positively chemotropic to the lower concentration of oxygen 

 at the center of the liquid. 



Once established, such an aggregation of flagellates does not remain 

 long in the same form. An area free from flagellates appears at the 

 center of the aggregation so that the organisms lie in a circular band 

 surrounding the clear area. The latter increases in size and its 

 bordering band of flagellates in diameter, the band gradually becom- 

 ing less circular and more square in shape, if the cover-glass is a square 

 one. The clear central area is a region where the oxygen consumption 

 of the flagellates has reduced the oxygen content to such a low value 

 that the organisms are forced to leave the region. They collect in 

 a band where the concentration of dissolved oxygen is an optimum 

 for them. It is the equilibrium position between the oxygen consumed 

 at the center and that diftusing in from the edges of the liquid. As 

 the consumption at the center is more rapid than the replacement 

 from the edge, the flagellate band moves outwards until it becomes 

 stationary at a position where the rates of consumption and replace- 

 ment of oxygen are equal. 



Although the flagellates collect in this manner in regions of optimum 

 oxygen concentration, yet greater concentrations of dissolved oxygen 

 have no injurious efi'ect on them. Concentrations of dissolved oxygen 

 lower than the optim.um have the effect of inhibiting the movement 

 of the flagellates. They recover their activity, however, immediately 

 they are given access to dissolved oxygen again. 



