Changes in Oral Apparatus Structure Accompanying Vacuolar Formation in the Macrostomal Form of Tetrahymena vorax. A Model for the Formation of Food Vacuoles in Tetrahymena1

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Abstract
The large cytopharyngeal pouch of the macrostomal form of Tetrahymena vorax, following the addition of calcium, can form a sealed, empty vacuole. The open cytostomal region of this cell, which averages about 16 μ in diameter, is closed by an upward (ventral) movement of the right and posterior ribbed walls, both of which project into the cytostomal cavity. At the same time, the anterior and left walls of the cytostome‐cytopharyngeal complex move to the right, forming a diagonally (right to left) placed furrow in the floor of the buccal cavity as these walls meet. As a result of the movement, the edges of the single membrane‐bounded cytopharyngeal pouch are brought together and fuse, producing the closed vacuole. Elements of the cytoskeleton appear to participate in the closure process. Three major groups of ribbed wall microtubules support the open cytostome. The anterior ribbed wall microtubules pass laterally along the anterior (dorsal) portion of the cytopharyngeal pouch to the left where they end in the specialized cytoplasm. Middle oral rib microtubules terminate at the right and posterior margin of the cytopharynx while microtubules from the most posterior region of the ribbed wall pass to the left terminating in the specialized cytoplasm. The fine filamentous reticulum, a striated reticulum that borders the right, posterior, and anterior margins of the cytostome‐cytopharyngeal complex, is in an ideal position to participate in these movements. It is anchored anteriorly high up in the buccal cavity to the cross‐connective between the third membranelle and the undulating membrane complex. It courses beneath the right and posterior ribbed walls and runs laterally along the anterior margin of the cytopharynx to the left side. Contraction or pulling of this reticulum would act to bring the microtubule‐reinforced walls of the cytopharynx together permitting fusion of the cytopharyngeal pouch membranes to form a sealed vacuole.