BEHAVIOR OF AMEBA TOWARDS VARIOUS SUBSTANCES. 319 



than in water. A grain of tyrosin that dissolves in water in ten 

 minutes remains in an ameba for over two hours without appre- 

 ciable diminution in size. This would seem to indicate that it 

 is not sufficient for tyrosin to go into solution in order to be 

 assimilated, but that it must be further broken down by digestive 

 action; unless indeed the assimilation of dissolved tyrosin goes 

 on very slowly. It is impossible to say at present why tyrosin 

 remains undissolved for so long in the ameba's body. Negative 

 behavior toward tyrosin is similar to that toward other sub- 

 stances. In one case negative behavior was changed to positive 

 behavior by presenting the ameba with a grain of globulin. 

 Before the globulin was ingested the tyrosin was avoided; after 

 ingestion, a grain of tyrosin was eaten. This is a very good 

 illustration of the possibility of habit formation in ameba. Tyro- 

 sin can be sensed at a distance of at least 125 microns. Although 

 it is quite likely that the ameba reacts to tyrosin in solution in 

 the experiments described above, yet the ameba invariably moves 

 with great accuracy toward the center of diffusion, the tyrosin 

 grain itself. The mere presence of molecules (or ions) in solution 

 would therefore not explain the whole behavior. Some other 

 factor must operate such as differences in the concentration of 

 the molecules of tyrosin in solution, as would occur in the process 

 of going into solution, for without some such additional factor 

 the ameba would be unable to find the solid tyrosin. 



Gelatin. Knox's Sparkling (not acidulated) commercial gelatin 

 was employed. Only a few tests were made, owing to the experi- 

 mental difficulty of handling it. One experiment only is recorded 

 in this paper. 



A small piece of gelatin was placed to the right of the path of 

 a granular ameba I. A small pseudopod which was extended 

 toward it came very nearly into contact with it, when the ameba 

 turned to the right, avoiding the gelatin 4. A small pseudopod 

 was then thrown out on the convex side of this pseudopod toward 

 the gelatin, but it was retracted before it came quite into contact 

 with the gelatin. The ameba finally moved on through the 

 vestige of the previous main pseudopod. 



Arrowroot Starch Paste. This was made by boiling starch 

 paste with water and allowing it to cool until a rather stiff gel 



