SPINDLE FORMATION AND DIVISION OF THE GIANT PRIMARY NUCLEUS OF ACETABULARIA (CHLOROPHYTA, DASYCLADALES)

The following cytological events are demonstrated, by light and electron microscopy, to be correlated with the division of the primary nucleus of various species of Acetabularia: (1) formation of a perinuclear cytoplasmic ‘dense zone’ (2) reduction of nuclear size from about 150 μm to 40 μ diameter; (3) reduction of the numerous nucleolar units to one persisting nucleolar structure; (4) formation of large nucleolar cavernae (‘vacuoles’); (5) condensation of nucleolar portions; (6) appearance of condensed chromosomes; (7) formation of a giant spindle within the apparent confinements of the nucleus; (8) elongation of the spindle apparatus during division from anaphase to telophase; (9) formation of two small daughter nuclei; (10) appearance of residual spindle structures in the internuclear region; (11) appearance of ‘redundant nuclear envelope’ structures apparently discontinuous with the nuclear envelopes of the daughter nuclei. A detailed analysis of the mode of spindle formation has been made by time lapse cinematography. It is concluded that the division of the giant primary nucleus of Acetabularia involves the formation of a special unusually large spindle which, however, is essentially similar to that described, e.g. in mitoses of other green algae. © 1979, International Society of Differentiation. All rights reserved.