no AMEBOID MOVEMENT 



solid and dissolved substances in the culture medium. But the 

 mechanism controlling the direction of the path of an ameba is 

 apparently much more delicate than that in paramecium, for it 

 is only occasionally that a considerable succession of regular 

 sinuosities are described by an ameba in moving over a flat sur- 

 face. On the other hand, a few fair curves are found in the path 

 of practically every ameba if carefully observed for an hour or 

 more under favorable conditions. 



To observe the path an ameba describes in moving over a flat 

 surface, the following conditions must be fulfilled. One must 

 have a small glass dish with a flat bottom, polished preferably, 

 but not necessarily, of the size of a small petri dish, but square 

 so as to fit into a mechanical stage. The dish should be filled 

 with culture fluid free from solid particles. Centrifuging 

 the culture medium, or dialyzing distilled water in the 

 culture medium, will yield a satisfactory medium. It is 

 only by experience that one can pick out an ameba that 

 seems to be in an optimum condition for this purpose, that is, 

 free from strong internal stimuli, such as those from a large 

 mass of food, etc. Just as we speak of "clean-limbed" athletes, 

 meaning thereby a high degree of muscular coordination, so one 

 who has worked with these animals for some time acquires the 

 capacity to pick out "clean-limbed" amebas ; though how these 

 differ from others is just as impossible to describe adequately as 

 to tell what a clean-limbed athlete is. But having selected two 

 or three amebas that move in a well coordinated manner and 

 passed them through two or three changes of water free from 

 particles, they are placed in the middle of a dish and allowed to 

 remain for ten or fifteen minutes before observations are begun. 

 A small shade should be placed in front of the dish if very strong 

 light can reach it. It does not matter if diffuse light reaches 

 the dish. A camera lucida with its appurtenances is absolutely 

 essential. In addition to the ordinary precautions the edge of the 

 paper must be laid parallel with the side of the mechanical stage, 

 for a number of sheets of paper will have to be used up in the 

 course of an hour or two and these must be pasted together 

 properly to reconstruct the path. The best magnification shows 

 the ameba two to five cm. long on the paper. Drawings should be 



