In the multinucleate cap rays of the green alga Acetabularia mediterranea the cell surface increases dramatically within a short time period during the final stages of coenocytotomic cleavage. In early stages of cyst formation the cytoplast is traversed by numerous large and prolate cleavage vesicles which are characterized by typical columellar or spinous coat structures. The cleavage vesicles are closely associated with the surface of plastids and, to a lesser degree, of mitochondria. This intimate association seems to be mediated by regularly spaced, densely stained intermembranous cross-bridge structures and is maintained throughout cleavage. These cleavage vesicles contain a finely fibrillar material structurally similar to the hyaline layer of mucilage that fills the space between the plasma membrane and cell wall. They line up with invaginations of the plasmalemma and vacuole membranes and, together with smaller vesicles interspersed, constitute preformed "perforation lines" for the final separation of the coenoblast portions. Equidistantly spaced plaques of attachment of such vesicles with surface membrane are described. We hypothesize (a) that the cleavage vesicle membrane is the immediate precursor to the new postcoenocytotomic surface membrane, (b) that the cleavage vesicle coat structures are integrated into the subsurface coat of the plasma membrane, (c) that growth of the laterally attached cleavage vesicles by intussusception of small fuzzy-coated vesicles is confined to their "free ends," (d) that the intermembranous cross-bridge elements are related to bristle coat structures and play a role in the establishment of the cleavage lines, and (e) that the coenocytotomic cleavage process is organized so that adjacent plastids are separated in a way that guarantees the inclusion of several plastids in each cyst.

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