MOVEMENT 29 



this movement. Actual projections from the sur- 

 face were only occasional and never marked, and 

 progression was very slow. The amoeba has also 

 become smaller on the average, a similar diminu- 

 tion in size being found to take place in a large 

 pond amoeba after cultivation for some time in 

 tap water. Possibly the comparative absence of 

 salts has something to do with this modification, 

 as Wherry noticed a similar influence on the 

 species of amoeba with which he worked. The 

 peculiar type of movement first observed was 

 found to alter after cultivation on salt-free agar 

 media, and has not been regained, although, on 

 jellies containing a high percentage of sodium 

 citrate, there is a distinct increase in size of the 

 whole cell. 1 



Movement. As far as we now know, the 

 character of the movements of an individual 

 amceba is determined both by the chemical and 

 the physical condition of its environment. The 

 same amceba exhibits different types of motion 

 according to the circumstances under which it is 

 examined. In a hanging-drop preparation, motion 

 is unrestrained and the body turns over and over, 

 rapidly exposing different aspects to the view of 

 the observer, and progression takes place partly 

 by means of this rotation. Resting on a smooth 

 surface (such as glass), progression becomes more 

 of a crawling character. On the surface of a firm 

 jelly the movement is slower, the pseudopodia are 



1 Minchin in " An Introduction to the Study of the Protozoa, 1912," 

 p. 198, refers to the effect of environment, viz. : " These experiments 

 . . . raise the suspicion that many species of marine Protozoa may be 

 only different forms, due to change of medium, of fresh-water species, 

 or vice versa" 



