The s itu a tio n in the elongate micronuclei (about 80 pm) o f _P. prim aurelia is very s im ila r to th a t described above fo r P. te tra u re lia
(Figs, 149 & 150), The diameter o f micronuclei varies along the lengths o f th e ir spindles. The la rg e st micronuclear diameter is in the spindle mid-region (about 0.8 pm) and tapers o f f to about 0.4 pm towards each
end o f a spindle. The number o f spindle microtubules is greatest in the spindle mid-region (about 190 microtubules) and le a st near each end o f the spindle (about 40 m icrotubules). Microtubule diameter also varies. Microtubules in the spindle mid-region are predominantly about 28 - 31 nm in diameter while those nearer each end o f the spindle are predominantly about 19 - 24 nm in diameter. Microtubules o f about 19 - 24 nm diameter extend in to the chromatin-containing region, and are associated with m ic r o fib r ils th a t are about 6 nm in diameter (Fig. 151).
(b) Cold treatment
The s e n s itiv ity and l a b i l i t y o f spindle microtubules to cold has been well documented (Inoue, 1964; Brinkley & C artwright, 1970, 1975; Lambert & Bajer, 1977). Not a ll spindle microtubules react s im ila r ly to cold (B rinkley & C artwright, 1970), I t is possible th a t micro tubules w ith d iffe r e n t diameters have d iffe r e n t l a b i l i t i e s . I f th is is the case, i t would provide fu rth e r evidence fo r the presence o f two classes o f spindle microtubules in the micronucleus. Organisms
destined fo r cold treatment were selected when cleavage furrows were f i r s t apparent. They were cooled to 0°C and subsequently fixe d using the procedure described in the M aterials and Methods. Such cold-treated organisms were s e r ia lly sectioned. A ll the spindle microtubules p e rs is t during cold treatment. Microtubule diameters are e ith e r about 31 nm or about 24 nm as in control organisms (compare Figs. 149 & 150 with Figs. 152 & 153). The intra n u cle a r m atrix is very densely stained (apart from a few 'empty' spaces; Fig. 152), and is concentrated close ly around the spindle microtubules (Fig, 155). Cross-bridges between microtubules are not apparent. Cold-treated organisms have a s im ila r micronuclear organ is a tio n to th a t in control ( i. e . untreated) organisms w ith regard to micronuclear diameter tapering towards the poles, and microtubule numbers
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and diameters decreasing towards the poles (Figs. 152-154). Microtubules th a t are about 24 nm in diameter extend in to the terminal knobs as in control organisms. However in cold-treated organisms densely-staining m aterial is clumped around the chromatin and microtubules leaving large e le ctro n -lu ce n t regions (Fig. 154). One e ffe c t o f the dense m atrix is to render the cores o f microtubules p a r tic u la r ly d is t in c t and th is h ig h lig h ts the differences in microtubule diameters especially c le a rly (compare Figs. 155 & 156).
In most c e lls most spindle microtubules are cold la b ile . This generalisation may not apply to c ilia t e s . For example, Williams & Williams (1976) subjected d iv id in g Tetrahymena to cold shock th a t was applied in the same way as th a t used here fo r Parameciurn. Micronuclear spindles were not examined, but in d iv id in g macronuclei, microtubules persi sted.
(c) Final stages o f micronuclear elongation
The chromatin-containing terminal knobs begin to separate from the spindle when micronuclei reach lengths o f up to 110 pm. The diameters o f micronuclei in the spindle mid-region are about 0.6 pm (Fig. 157). Micronucleardiameters taper from the mid-region towards each end o f the spindle (Figs. 158 & 159). Near terminal knobs, micronuclear diameters are about 0.4 pm (Fig, 159). In the spindle mid-region there are about 130 microtubules. Most o f these are about 31 nm in diameter. The number o f m icrotubules/cross-section decreases on e ith e r side o f the spindle mid-region as terminal knobs are approached. Sections near terminal knobs show th a t microtubules are nearly a ll about 28 - 31 nm in diameter.
DISCUSSION
In his recent review o f mitoses in lower eukaryotes. Heath (1980) points out th a t 'nothing is known about spindle development in
Paramecium' nor how microtubules are arranged in the 'e n tire nucleus'. What has been described are the gross micronuclear shape changes, and i t has been established th a t there are no polar c e n trio le s , the spindle is m icrotubular, the nuclear envelope remains in ta c t throughout
d iv is io n , and th a t kinetochores are present (Jurand & Selman, 1969; Stevenson & Lloyd, 1972).
In th is analysis, data from micronuclei a t metaphase, and e a rly and la te stages o f anaphase spindle elongation have been compared.
P a rtic u la r a tte n tio n has been paid to changes in the d is tr ib u tio n o f
microtubules and other cytoskeletal elements w ith in elongating m icronuclei. Only the d ivid in g micronucleus o f Tetrahymena has been studied in
comparable d e ta il (LaFountain & Davidson, 1980).