Functional interdependence of pseudopodia in Amoeba proteus stimulated by light-shade difference.

Polytactic cells of Amoeba proteus were exposed to localized photic stimulation. When a pseudopodium is stimulated to advance, by shading it, other pseudopodia are retracted. Activation of the shaded front is the primary response, and contraction of other fronts the secondary one. When a pseudopodium is inhibited by illuminating its frontal segment, or when it is allowed to enter the bright zone in the course of migration, it slows down and stops but its eventual retraction depends on the existence of other possible directions for the endoplasmic flow. Therefore, if other active pseudopodia are lacking, the front suppressed by light cannot retreat effectively until new fronts arise in other body regions kept in shade. In all experimental situations the development of new fronts or the activation of forward flow in lateral pseudopodia precedes the contraction of the former leading pseudopodium. Also the reversal of direction of the endoplasmic streaming begins at the new front, and then it gradually extends until it reaches the former front. The results confirm the interdependence of different pseudopodia in the same individual and they contradict the concept that pseudopodia behave as separate functional units. On the other hand, they indicate that formation of new pseudopodia should not be explained as a simple secondary effect of contraction of the older ones but, on the contrary, as a phenomenon that initiates the changes in the pattern of flow in amoeba. The general interpretation is based on this variant of the pressure-flow theory of amoeboid movement, which attributes the motive power to the contractile activity of the whole cell cortex and the steering role to events taking place in the front of the migrating cell.