MOVEMENTS OF AQUATIC MICRO-ORGANISMS 307 



stick but always remains in a fine granular condition ; it has, 

 moreover, the advantage of being black, so that the figures 

 formed by it can be easily seen. 1 got good results when this 

 precipitate was allowed to settle in water but much better 

 when it was allowed to settle in a denser liquid, such as a 

 solution of one part of glycerine in four parts of water, by 

 which its relative density to the glycerine solution was nearly 

 equal to that of the Euglense to water." 1 



" If a flat glass dish, which can be covered with a glass 



glate to exclude air, similar to that used in experiments with 

 uglenae, is filled with water containing a precipitate of osmic 

 dioxide, the separate particles settle down in straight lines and 

 form a thin film on the bottom. If the dish is now gently 

 turned upside down, this film of particles, in a second or two, 

 again begins to sink in the water but directly gravity begins 

 to act upon them they become attracted into a network (fig. 5) 

 almost exactly resembling the network produced during the 

 Euglena aggregations under similar conditions (fig. 2). 



" If the downward movement is retarded by the use of a 

 denser solution, such as glycerine and water, the network which 

 is at first formed (fig. 5) is not only much clearer but, under 

 such conditions, tends to break up into separate masses or 

 groups of particles, exactly resembling a later stage in the 

 aggregation of Euglena. The resemblance extends even to the 

 darker, denser central stream surrounded by a less dense 

 peripheral layer. This less dense peripheral layer consists of 

 a more or less well-formed vortex ring produced by the friction 

 of the water. In the case of Euglena, the lighter area around 

 the denser central mass is also of the nature of a vortex ring, 

 but is not so clearly marked owing to the upward movements 

 of the organism. 



" Except for the fact that the cells ol Euglena are capable 

 of free movement, the aggregation of the particles of a pre- 

 cipitate appears to be similar to the aggregation of living 

 Euglenae." 2 



" The aggregation of these various sediments appears to 

 depend upon the fact that a thin film of particles, together with 

 the surface layer of water in which they are contained, acts as 

 a liquid heavier than water. The aggregation is brought about 

 therefore not by the action of gravity upon the separate particles 

 but upon the mixture of particles and water as a whole, just 

 as would be the case if we were dealing with a homogeneous 

 liquid heavier than water." 3 



"The physical forces which are called into play during the 

 aggregation of a layer of sediment in a closed shallow vessel 



1 Wager, loc. at., pp. 372-3. 



2 loc. at., pp. 373-4- 

 * loc. cit., p. 374. 



