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Notes: SYNOPSIS. In axenic culture, the polymorphic ciliate, Tetrahymena patula, consists of two cell types, a small mouthed form (microstome) and a larger carnivorous macrostome both of which possess the tetrahymenal-type buccal apparatus. With the exception of over-all size, only minor differences were found in the structure of the oral area of the two cell types.Each of the membranelles, which lie on the left wall of the buccal cavity, consists of three long rows of kinetosomes with attached cilia. A series of parallel fibers originating at the base of the inner row of kinetosomes in each membranelle runs toward the adjacent, lower membranelle or in the case of the third membranelle toward the junction of the left and right walls of the buccal cavity. After a short distance, a portion of each fiber rises and subdivides to form a periodic array of fine fibrils which lie immediately under the pellicle and run parallel with the long axes of the membranelles. The remaining portion of these kinetosomal fibers continues through the cytoplasm to connect with the kinetosomes of the next membranelle.The undulating membrane, which borders the right wall of the buccal overture, consists of a single row of cilia. Examinations of thin sections show, however, that there are actually two rows of kinetosomes in the membrane base. The outer row is ciliated while the inner row of kinetosomes, which are alternately spaced relative to those in the outer row, lies deeper in the cytoplasm and has no cilia. Fibers project straight down from the base of kinetosomes in both rows and join to form a tangled network which underlies the entire length of the membrane.The oral ribs begin near the base of the undulating membrane and descend in decreasing number to the cytopharynx. There is a connection between the fiber system of the undulating membrane and the tips of the oral ribs. The ribs appear in cross section as raised, thickened ridges of the inner pellicular layer with the outer layer adhering only at the top of the ridges. Immediately underlying the oral rib area is a relatively thick differentiated layer composed of fine fibers within a ground substance.