306 SCIENCE PROGRESS 



oxygen given off by the organisms themselves is not the con- 

 trolling cause seems to be proved by the fact that the aggregation 

 takes place best in the dark, when we may suppose the Euglenae 

 are not giving off oxygen. The presence or absence of carbon 

 dioxide in the liquid appears to make no difference in the 

 aggregation. Jennings : states that the crowding of Paramecium 

 is due to the chemotaxis exerted by the excreted carbon dioxide. 

 I have not been able to obtain any evidence that this is the case 

 with Euglenae. Various experiments tried with it gave purely 

 negative results. Bubbles of carbon dioxide introduced into the 

 tubes or vessels containing motile Euglenae seemed to have no 

 effect ; there was no accumulation of Euglenae around them as 

 in the case of the oxygen bubbles and no modification in the 

 aggregation. The only effect of the carbon dioxide that I have 

 been able to detect is that in its presence the Euglenae remain 

 motile for a longer time than in ordinary air or oxygen, in which 

 they tend to become rounded off." 2 



It is, moreover, very difficult to suppose that variations of 

 this kind in the liquid would produce a regular grouping of 

 Euglenae and a persistent cycle of up-and-down movements. 



" The fact that the organism is heavier than water and that 

 the downward movements are due to the mechanical action 

 of gravity suggested the possibility that the network-like 

 grouping and aggregation might also be a purely physical 

 phenomenon due to surface tension and cohesion and it oc- 

 curred to me that the behaviour of chemical precipitates and 

 other fine sediments of various kinds might afford some clue 

 to an explanation of the phenomena. 



" It was necessary, of course, to obtain a precipitate as 

 nearly like the Euglenae cells as possible as regards density 

 and absence of a tendency to become flocculent. Various 

 precipitates were tried, as well as sediments of different kinds, 

 such as starch grains, dead cells of yeast and Chlamydomonas, 

 fine coal dust, manganese dioxide, etc., all of which were useful ; 

 but the best results were obtained with dead cells of Chlamy- 

 domonas killed in osmic acid and a precipitate of osmic dioxide 

 (Os0 2 ) produced by adding a dilute solution of ferrous sulphate 

 to the ordinary i per cent, solution of osmic acid used for 

 fixing. This precipitate does not become flocculent and does not 



1 Jennings, H. S., "Studies on Reactions to Stimuli in Unicellular Organisms. 

 I. — Reactions to Chemical, Osmotic, and Mechanical Stimuli in the Ciliate 

 Infusoria," Journ. Physiol. 21, 1897. 



" Reactions to Stimuli in Unicellular Organisms," Amer. Journ. Physiol. 3 

 1899, 1900. 



Behaviour of the Lower Organisms, New York, 1906. 



1 Wager ; loc. cit., pp. 371, 372. 



