l6o THE BEHAVIOR OF LOWER ORGANISMS. 



nearly backward (Fig. 51, c). This change of position is due to the 

 contraction of the posterior part of the Amoeba. The ectosarc just 

 behind the base of the pseudopodium contracts toward the middle, as 

 described on page 171. As a result the pseudopodium must swing 

 around till it points nearly backward. The mechanism of the process 

 will be best understood by an examination of Fig. 51, x. 



Finally, what remains of the pseudopodium reaches the posterior 

 end or tail (Fig. 51, d). By this time usually all that is left of it is a 

 small roughened projection, its surface being of essentially the same 

 character as that of the tail. This projection fuses completely with the 

 tail, its projections taking up a portion of the surface of the latter. The 

 tail is in fact nothing but the fused remnants of all the pseudopodia 

 that have been formed, together with the contracted outer layer of the 

 body of the Amoeba (the latter cannot be distinguished in any essential 

 way from a pseudopodium). A roughened tail is formed de novo when- 

 ever an Amoeba suddenly changes its direction of movement. The 

 previous anterior end then becomes roughened in contracting and forms 

 a typical tail. This latter unites with the old tail if any of the latter 

 remains. The substance of the tail gradually passes forward into the 

 rest of the body, as we have seen. 



MOVEMENTS AT THE ANTERIOR EDGE. 



As in Amoeba verrucosa and its relatives, so in the species of more 

 changeable form, the most active movements are taking place at the 

 anterior edge. In Amoeba proteus and A. Umax one sees still more 

 distinctly than in the species before named the pushing forward of a 

 series of waves of hyaloplasm which become attached to the substratum 

 in front. In Amoeba Umax and its relatives especially such a wave 

 may be very pronounced, extending forward at times one-fifth the 

 length of the body or more, though usually much less. 



At first the advancing wave, as it moves rapidly forward, is usually 

 free from granules, and may be spoken of, therefore, as hyaloplasm. 

 If the motion is less rapid, however, it contains granules, and is not 

 distinguishable in any way from the interior endosarc. Where it is at 

 first free from granules, it is nevertheless highly fluid in character, as 

 is shown by the fact that it flows and spreads out swiftly, and that the 

 granules of the endosarc pass into it rapidly. The freedom of the 

 advancing hyaloplasm from granules is not due to its greater density 

 or solidity as a result of the action of water upon it, as has sometimes 

 been maintained, for it is at first free from granules ; then, after the 

 water has acted longer upon it, it becomes filled. Apparently the 

 reason for its freedom from granules is merely the fact that it moves 

 forward faster than the granules and leaves them behind. This view 



