Notes: Summary Details of the development of the cortex and of its cytoskeletal components prior to cell division are described forCyclidiopsis acus. The description is based on transmission and scanning electron microscopy (SEM) of semi-serial thin sections of cells at progressive stages of division. Four microtubules (Mt) having precise positions, are associated with each cortical strip. In the interphase cell, however, only two microtubules are associated with each strip or reduced strip within the flagellar canal, the other two being evident only when the strips pass over the external surface of the body. There appear to be organizing centres in the form of dense material extending along the length of MT 2 and MT 3 of each band. The development of each new cortical strip begins as an outgrowth (the “ridge”) in the region of the organizing centres where MT 2 and MT 3 of the new cortical strip appear. The “hook” edge of the epiplasmic layer of each new strip forms first, then the strip extends tangentially to the cell surface, pushing itself between the old strips. The process begins at the anterior part of the cell and extend backwards. As this happens, the two other microtubules (MT 1 and MT 4) appear. In the interphase cell the strips within the flagellar canal are of two sizes, 16 major strips alternating with 16 minor ones. The number of strips decreases progressively toward the base of the canal. Each strip has two microtubules, possibly corresponding to MT 2 and MT 4 of the cortical strips. During division, the number of canal microtubulepairs increases and doubles, each new strip receiving a pair of microtubules. The old minor strips grow to the same size as the major strips. The process of strip duplication leads to 32 major old strips and 32 minor new, intercalated strips. Cell division begins with the formation of a cytoplasmic ridge between the two pairs of kinetosomes. The ridge grows to form a partitioning wall, making two reservoirs each containing a flagellar apparatus and three kinetosome associated microtubular rows. A pinching off takes place between the two dorsal rows of paired microtubules at the base of the canal. This zone, marked by additional microtubules, progresses toward the canal aperture and divides the canal in two. The cleavage furrow, which contains additional sets of microtubules, begins at the aperture and progresses posteriorly. Strip duplication precedes cell division. Of the three sets of microtubular rows associated with the kinetosomes, two are clearly involved in cell division. The dorsal row, associated with the paired microtubules of the cortical folds in the flagellar pocket, extends to the outer cortex (MT 2 and MT 4). The ventral row is associated with the microtubular bands which develop in the partitioning wall at the base of the reservoir in dividing cells and then forms the cryptic cytopharynx. The intermediate row is probably involved in the formation of additional microtubules in the cleavage furrow of the flagellar canal and the cortex.