The Evacuation Mechanism of the Water Expulsion Vesicle (“Contractile Vacuole”) of Amoeba proteus*

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Abstract
SYNOPSIS.Frame by frame analysis of cinephotomicrographs of the evacuation of the water excretory vesicle ofAmoeba proteusshows that this organelle (commonly called the “contractile vacuole”) does not contract as the water is expelled. Instead, pressure exerted by the underlying endoplasmic “gel” pushes the vesicle against the plasmalemma, bulging the latter until a pore opens through it. The cell membrane and apposed vesicular membrane both rupture through the plasmalemmar pore; the fluid rushes out; and the vesicle collapses against and inverts into itself, but does not contract. There is therefore no systole of the vesicle due to any contraction of it. A survey of the literature shows that others who have studied the evacuation of these water excretory vesicles in amebas, suctorians, and ciliates have also described them as collapsing, not contracting. Recent electronmicrographs of a number of protozoa also indicate that the water excretory vesicle collapses, and does not contract as it evacuates its contents; and that there are no fibrils surrounding the organelle which might promote its contraction. We suggest that the term “contractile vacuole” be discarded as a descriptive and identifying term for such organelles; and that they be called “water expulsion” organelles. We further suggest that the term “vacuole,” which implies that the structure is empty, be replaced by the termveside, implying its function as a small container, and that the organelle be preferentially called thewater expulsion vesicle.We also suggest that the terms “systole” and “diastole” be discarded and be replaced withenlargementandevacuationso that the erroneous implication of a contraction and relaxation cycle may be dispelled.The succeeding new water expulsion vesicle in Amoeba proteus seldom forms at the site of the old one, but usually forms at a new site in the sol anterior to the gel of the tail region, to which it is transferred prior to expulsion. A tentative suggestion of how this transfer may occur is offered.