Ultrastructure of the frontal cap of monotactic forms of Amoeba proteus.

The frontal cap of the monotactic form of Amoeba proteus is separated from other cell components by a continuous structure defined as the "membrane-like envelope" (MLE). It originates from the membranes of cytoplasmic vesicles and vacuoles. The border zone between the cap and the cytoplasm is strongly vacuolized. Structural differences between frontal caps, depending on the degree of their development, indicate that the growing cap gradually fills up the whole tip of an advancing pseudopodium, and at the front it reduces the cortical layer in the interstice between the MLE and the outer cell membrane, up to its eventual disrupture. This is probably the efficient cause of the specific morphological and motory pattern of monotactic amoebae. These results and conclusions are supported by an ultrastructural analysis of the artificial frontal caps obtained by injecting oil droplets into polytactic cells, a procedure transforming polytactic forms into forms morphodynamically analogous to the natural monotactic amoebae.