The lobopodial pseudopods of the shelled amoeba Difflugia undergo a rapid active contraction and length shortening which results in movement of the organism, or rapid pseudopod retraction. In polarized light, the pseudopod shows high birefringence before and during this normal contraction, suggesting a high degree of linear organization, and a complex pattern of changes. Immediately conventional fixation begins, pseudopods retract rapidly, and show changes in birefringence. When longitudinal sections are viewed in the electron microscope, such fixed, contracting pseudopods are seen to contain thick (14-16 nm) and thin (5-8 nm) microfilaments. Montages demonstrate that these microfilaments are found in close parallel association with each other, and lie parallel and peripheral to the longitudinal axis of the pseudopod, which is the axis of contraction. This distribution suggests that the patterned microfilaments could be involved in the shortening process and that they could account for the birefringence seen in the contracting pseudopod. Rapid alignment, or assembly, is also suggested.

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