THE MOVEMENTS AND REACTIONS OF AMCEBA. 1 83 



a depression in the ectosarc that may last for some time (Fig. 66). 

 (We will suppose that the thrust does not detach the Amceba from the 

 surface, as sometimes happens.) At once the anterior portion of the 

 Amoeba ceases to advance. It remains quiet for a definite interval, 

 which I should judge to be about a second, while the current from behind 

 continues to move forward. As a result of the stoppage at the anterior 

 edge there is a heaping up of the protoplasm in the middle of the body. 

 After about a second the part stimulated begins to contract and currents 

 start backward from it. Thus the currents from the two ends meet in 

 the middle, often producing a further heaping up in this region. Usu- 

 ally, however, the ectosarc of one side of the Amoeba quicklv gives 

 way and a new pseudopodium starts out laterally. As a rule this new 

 pseudopodium is formed near the original anterior margin, often at the 

 very edge of the area directly affected by the stimulus (Fig. 66). The 

 reason for this is evident. Only this anterior half of the Amoeba is 

 expanded and attached to the substratum, the posterior half being free 

 and contracted. It is, therefore, much easier to continue locomotion 

 by sending out pseudopodia somewhere in the attached region than 

 behind it. If sent out in the unattached region, the original contraction 

 would have to be overcome, and no locomotion could occur until the 

 pseudopodium had (by chance.'') come in contact with the substratum 

 and become attached to it. By sending out pseudopodia thus in some 

 portion of the attached region, the movement is, in a certain sense, a 

 continuation of that which was taking place before stimulation, though 

 in a different direction. The Amoeba follows a path which forms an 

 angle with its previous one. 



The course of the reaction may vary considerably from that above 

 described. If the stimulus is weak the reaction may consist merely in 

 a stoppage at the point stimulated without any contraction there. The 

 current from behind continues ; a pseudopodium breaks out at one side 

 of the region stimulated, and the Amoeba moves in the direction so 

 indicated. If the stimulus is very weak the current may cease only 

 for an instant in the region stimulated, then continue as before ; the 

 direction of progress thus remains unchanged. 



If the stimulus is very strong the contraction which takes place at 

 the region stimulated may be very marked, resulting in the formation 

 of strong folds in this region. The contraction may include the entire 

 anterior end of the Amoeba. Such a contraction destroys the attach- 

 ment to the substratum, and the new pseudopodium now bursts out in 

 some part of what was the posterior end of the body. The new course 

 followed may then be at right angles to the old one, or at any greater 

 angle, or the course may be exactly reversed, the new pseudopodium 

 being formed at the posterior end. If the posterior end was much 



