488 SPONTANEOUS AGGREGATION OF FLAGELLATES 



all. Furthermore, a flagellate band which has been caused to retire 

 inwards by keeping the preparation at a low temperature will move 

 outwards again as soon as the slide is replaced in the higher temper- 

 ature of the laboratory. 



The Cause of Aggregation and Band Formation. 



In seeking for the causes of these phenomena, there are three 

 questions to be asked. 



1. What is the cause of the aggregation of the flagellates at the 

 center of the preparation? 



2. Why does an aggregation become a band surrounding a region 

 clear of flagellates, which continuously increases in size? 



3. Why does the central clear region cease to increase in size when 

 the band of flagellates bordering it has reached a certain distance 

 inside the air-water surface at the edge of the cover-slip? 



The different condition which arises at the center of the preparation, 

 •■attracting thither the flagellates, must be due to some substance or 

 :substances produced by the organisms themselves. It cannot be 

 any excretory product of the nature of a solid in solution or a liquid, 

 for, since the flagellates were originally evenly scattered throughout 

 the preparation, the newly produced substance too would be evenly 

 distributed and would tend neither to attract nor to repel the organ- 

 isms in any particular region. The changed condition in the center 

 of the preparation must be due to some volatile substance in solution, 

 which at the free edges of the liquid is effecting an exchange with 

 the atmosphere, either going into or out of solution. The only gases 

 in solution which would be changed in amount by the living organisms 

 are oxygen and carbon dioxide; the amount of the former present in 

 solution in the water must continuously decrease and the amount 

 of the latter increase. These changes in the concentrations of dis- 

 solved oxygen and carbon dioxide will take place more rapidly in the 

 central region of the liquid than at its edges; for at the edges the 

 oxygen used up by the flagellates will be replaced from the atmos- 

 phere, while the carbon dioxide produced by them in this region will 

 go out of solution into the atmosphere as soon as its tension in solu- 

 tion rises above the value corresponding to its partial pressure in the 



