THE MOVEMENTS AND REACTIONS OF AMOEBA. 177 



restore them at once to their original form, resulting in a return of the 

 whole body to its original form. 



If, then, we hold that the substance of Amoeba is composed of such 

 an alveolar structure, the above observations are intelligible, even 

 though no part of the substance of the organism is in the solid state. 

 But whatever the cause, we must recognize that the protoplasm of 

 Amoeba shows in the gross some of the characteristics of solids. 



Leaving out of account the minute structure of the protoplasm, most 

 or all of the observations detailed above could be accounted for on the 

 assumption that the body of Amoeba consists of a sac of fluid, the outer 

 wall of the sac being tough and elastic, and, as is shown later, contrac- 

 tile. In this case only the outer layer, the ectosarc, would show the 

 characteristics of matter in the solid state of aggregation. Certain 

 observations made by the writer indicate that this is the true state of the 

 case. These observations are as follows : In several cases a long, slender 

 pseudopodium, formed of both endosarc and ectosarc, was stimulated at 

 the tip, causing the endosarc to be withdrawn, and leaving the pseudo- 

 podium formed of ectosarc alone, as illustrated in Fig. 50, page 158. Such 

 pseudopodia could with the glass rod be bent sharply at an angle, and 

 would often remain thus for some time. If, while thus sharply bent, 

 the endosarc, as sometimes happens, begins to flow back into the 

 pseudopodium, the latter straightens out with a sort of jerk as soon as 

 the endosarc begins to fill it. The ectosarc thus acts like an empty 

 glove-finger, which might bend over when empty but which would 

 straighten out on becoming filled with a fluid. A tough skin could 

 perhaps be formed by an alveolar fluid in accordance with the princi- 

 ples developed by Rhumbler as above set forth. Whatever the explana- 

 tion, the experiments indicate the existence of this tough skin-like layer 

 on the outer surface of the body. 



CONTRACTILITY IN THE ECTOSARC OF AMCEBA. 



Besides elasticity of form, the outer layer of Amoeba clearly has the 

 power of contracting locally. This is a fact which is omitted from 

 consideration in many of the theories in which amoeboid movement is 

 referred to local changes in the surface tension of a fluid mass. It will 

 be well, therefore, to set forth some of the observations on this point. 

 I transcribe here some of my notes, with the corresponding sketches. 



1. Specimen with a single long, prominent, curved pseudopodium. 

 This rather quickly swings around bodily toward its concave side, and 

 unites with the protoplasm of the body (Fig. 62, a). 



2. A specimen sends out a long, curved pseudopodium (Fig. 62, &). 

 This slowly straightens out, passing from position i to position 2. 



3. A. angulata usually sends out at the anterior end a single 

 pointed pseudopodium obliquely upward into the water (Fig. 62, c) . 

 This point frequently waves from side to side like an antenna. 



