THE MANIFESTATIONS OF LIFE 



75 



to appear to have its substance continually flowing for- 

 ward in the direction of progress. 



As the moving amoeba is watched it seems to be un- 

 certain as to the best course to pursue so that the an- 

 terior edge is not uniformly extended, but commonly 

 flows out into elongate rounded processes, the pseudo- 

 podia, one of which becomes larger and larger as the 

 cytoplasm flows into it, while the remainder are gradu- 

 ally withdrawn. 



Progress is effected with great slowness, and through 

 an unending series of changes in the shape of the 

 organism. 



X X* 



FIG. 21. Diagram of the movements in a progressing amoeba in side view. 

 A, anterior end; P, posterior end. The large arrow above shows the direction 

 of locomotion; the other arrows show the direction of the protoplasmic currents, 

 the longer ones representing more rapid currents. From a to x the surface is 

 attached and at rest. From x to y the protoplasm is not attached and is slowly 

 contracting, on the lower surface as well as above, a, b, c, successive positions 

 occupied by the anterior edge. As the animal rolls forward, it comes later to 

 occupy the position shown by the broken outline. (Jennings.) 



Locomotion is quite free in the amoeba, but cells may 

 lack locomotory power and still be amoeboid; i.e., capable 

 of changing their shape. Thus many of the radiolaria 

 and foraminifera being inclosed in mineral shells can- 

 not move from place to place, though they commonly 

 extend pseudopodia, which are sometimes extremely 

 long and delicate, through the minute openings of their 

 shells. 



The cells of the more simple metazoa retain a limited 

 amoeboid movement, and in the highest animals amoe- 

 boid cells may still be found. The best known of these 

 is the white blood corpuscle (polymorphonuclear). 



