318 A. A. SCHAEFFER. 



stratum and rolled over, contracting antero-posteriorly at the 

 same time 107. The effect of the tyrosin on this ameba was 

 similar to that on the previous ameba, except that the tyrosin 

 did' not seem to act so intensely or so quickly on the latter 

 ameba. A small grain of tyrosin T2, Fig. 109 was then laid 

 on the ameba but no change of behavior was observed. An- 

 other grain of tyrosin was then laid jn contact with the ameba 

 at the anterior end 112. The ameba sent out a pseudopod at 

 the anterior end, upward into the water, and out of contact with 

 the tyrosin ; but the weight of it became so great that the ameba 

 keeled over and so was removed from contact with the tyrosin. 

 Another grain of tyrosin was then placed in the path of the 

 ameba 113. The ameba moved on over it with very slight 

 change of behavior. Another grain of tyrosin was placed in the 

 ameba's path 117 but after slight movement toward it the 

 original direction of movement was resumed and the ameba 

 moved on without further change of behavior toward the tyrosin. 

 The tyrosin grain which this ameba ate remained in it for over 

 two hours without apparent reduction in size. 



Summary of Reactions toward Tyrosin. The behavior toward 

 tyrosin is anomalous; no other substance, so far as known, 

 induces similar behavior. Although the ameba seems to be 

 strongly attracted by the tyrosin, yet when the food cup is about 

 to close up the ameba withdraws. The negative reaction is due 

 apparently to too intense stimulation. In the case of one ameba 

 the impulse to withdraw did not make itself felt until the tyrosin 

 grain was eaten. The formation of the food cup also is peculiar. 

 Only when stimulated with tyrosin is the food cup formed by a 

 hollowing out of the under side of the pseudopod. No other 

 substance has been observed to produce such behavior. In at 

 least two instances the food cup was completed although the 

 tyrosin grain was not in it. This indicates that the formation of 

 food cups is somewhat of the nature of a reflex. The closure of 

 the food cups in the circumstances described above could not be 

 explained adequately by assuming that it was due to the direct 

 effect of the tyrosin in solution upon the surface tension or other 

 physical property of the ameba, as Rhumbler ('98, '10) contends. 

 Tyrosin dissolves in the body of the ameba much more slowly 



