But where do the particles come from? At exactly what regions of the ameba do they start to travel toward the anterior ends of the ameba? In _sphaeronucleosus_ and its congeners, it is very difficult to determine just when the particles begin to move toward the forward edge. Particles near the posterior end on the upper surface of these amebas moved forward slowly, much more slowly than particles near the middle. Sometimes particles near the posterior end seem to be motionless for some time, but the incessant though slow kneading process going on at the posterior end makes accurate observation difficult. Only in a general way it may be stated that particles begin their forward march at or near the posterior end. In amebas that habitually form pseudopods more accurate information can be obtained. In _proteus_ or _discoides_, for example, projecting pseudopods are often suddenly stopped and retracted, with a resultant change of an anterior to a posterior end. Particles attached to the outer surface on such pseudopods move toward the anterior end, of course, as long as the pseudopod is building, in the manner described in the preceding pages. But when the endoplasmic stream is arrested, the forward movement of the particle likewise stops. When the endoplasm starts to flow back into the main body of the ameba, the particle also starts moving back; but there is a period of a few seconds after the endoplasmic stream is reversed during which the particle remains quiet. And when it does start in to move, it moves only slowly. Within a few seconds, however, the average speed of movement is attained. This is true of particles located some distance away from the tip of the pseudopod. If the particle has reached the tip of the pseudopod before reversal of the endoplasmic stream takes place, the particle often remains at the tip until the pseudopod is almost completely withdrawn into the main body of the ameba (Figure 26, p. 60). At other times such a particle becomes displaced, presumably by irregular retraction of the tip of the pseudopod, and finds itself at the side of the pseudopod. When this happens it moves slowly toward the main body of the ameba, but faster than the tip of the pseudopod does.