The diagrams to the left of the figure show corresponding stages in the division of the macronucleus and one of the micronuclei. The macronuclei (black) have been drawn to the same scale as the profiles of the whole cell and their positions are accurately represented. Stages A-D in the development of the cleavage furrow are indicated in the stippled drawings (right), which show profiles of Nassula, after fixation and embedding, viewed from the aboral surface, with the anterior end towards the top of the figure. They were examined with a Philips EM 200 electron microscope operated at 60 kV with a 25-μ objective aperture.

The sections were picked up on grids coated with celloidin and carbon films and double stained with uranyl acetate ( Gibbons & Grimstone, 1960) for 90 min followed by lead citrate ( Reynolds, 1963) for 2 min. Blocks were trimmed accordingly and thin sections were cut with glass knives on a Huxley ultramicrotome. The orientation and fission stage of individual Nassula selected for sectioning were ascer tained by examination with a phase-contrast microscope after embedding. Dehydration in ethanol was followed by 3 changes of propylene oxide and flat embedding in Araldite ( Luft, 1961). Each plaque (approximate diameter 1 cm) contained about 100 Nassula and was handled in the same way as a piece of tissue. After washing once for 5 min as described above, Nassula were embedded in small flat plaques of 2 % agar, essentially following the instructions of de Haller, Ehret & Naef (1961). Washing for 12 h (7 changes) in the same phosphate buffer with 2 % sucrose was followed by post-fixation for 30 min in 1 % osmium tetroxide with 4 % sucrose and phosphate buffer (10 mM). Organisms were concentrated by mild centrifugation and fixed for 30 min at room temperature with a fixative containing 2♵ % glutaraldehyde ( Sabatini, Bensch & Barrnett, 1963) and 0♶ % sucrose ( Caulfield, 1957), buffered at pH 7♸ with phosphate buffer (I8ITIM). The dividing Nassula that have been examined were obtained from 1-day-old cultures starving Nassula from 6-day-old cultures were also examined to ascertain the fine structure of their interphase nuclei. In Paramecium each separation spindle has also been referred to as a middle piece (Mittelstiick) or connecting strand by Hertwig (1889) and Calkins & Cull (1907), respectively. Their formation has been described in several orders of ciliates, for example in the entodiniomorph Opisthotrichum ( Dogiel, 1925) and the hypotrich Euplotes ( Turner, 1930). Similar long micronuclear spindles have been appropriately called separation spindles in the hymenostome Paramecium by Wenrich (1926). In this organism a long central spindle is situated within each late anaphase micronucleus between the separating chromosomes ( Raikov, 1962). The possibility is considered here that the two changes referred to may facilitate the distribution of equal numbers of nuclei to each daughter Nassula.

Unlike other cells, some ciliates segregate several nuclei of two distinct types in a regular and precise manner during cell division. The former change has been described in several ciliates but the latter has not been reported before. Particular attention has been directed to two changes: the remarkable elongation of late anaphase micronuclei, and the interconnexion of the envelopes of all the nuclei just before completion of fission. Some of the possible functions of these changes as well as some of the possible developmental mechanisms responsible for them are discussed. This paper is mainly confined to a description of structural changes in the spindles and the nuclear envelopes of the dividing macronucleus and micronuclei of the ciliate Nassula during binary fission. Large numbers of ribosome-like granules are attached to the outer surfaces of the micronuclear envelopes where they sheathe the separation spindles. In long, late anaphase micronuclei, pores are abundant in the parts of the envelope surrounding the ends of the nuclei but are rarely found elsewhere. The nuclear pores of both types of nuclei are partly filled with dense material. During late anaphase a long central spindle, called here a separation spindle, is situated inside eachmicronucleus and elongates between the separating chromosomes. At metaphase a polar vesicle is situated at each pole of the micronuclei but centrioles are absent. Three types of filamentous structure have been distinguished in the spindles: microtubules of diameter 210–240 Å, fine branching filaments, and filaments which have a C-shaped transverse profile and a diameter of about 240 Å, for which the term C-filament is suggested. At one stage all the nuclei are linked together by the formation of membrane bridges in continuity with the outer membranes of their envelopes. The nuclear envelopes remain intact throughout nuclear division. During binary fission intranuclear spindles are formed in the three micronuclei and in the macronucleus.

Non examples of binary fission