THE NUCLEAR-APPARATUS OF THE DITRANSVERSAL CILIATE HOMALOZOON-VERMICULARE (CILIOPHORA, RHABDOPHORA) DURING INTERPHASE AND DIVISION .2. THE MICRONUCLEI AND SOME OBSERVATIONS ON CONJUGATION

The nuclear apparatus of H. vermiculare consists of a single monilifor m macronucleus and about 25 micronuclei. The micronuclei are about 3 m um in diameter and characterized by a meshwork of thick condensed chro matin. Mitosis is intranuclear and acentric as in all other ciliates. In metaphase, interpolar and chromosomal microtubules are abundant and the length of the micronuclei increases to about 5 mum. In late anaph ase, interzonal microtubules become prominent and the spindle elongate s to about 50 mum. In meta- and anaphase, the microtubules of the spin dle are attached to the polar vesicles, and in anaphase, chromosomes b ecome attached to it. In contrast to most other eukaryotes, micronucle ar mitosis is not strictly bound to cell division in H. vermiculare. W hile most of the micronuclei divide prior to cytokinesis, others retai n their interphasic shape or degenerate. In addition, some micronuclei divide in the interdivision period, i.e. between two successive divis ions of the cell and macronucleus. Mating cells of H. vermiculare beco me joined to each other in the cilia-free region covering the cytostom e. In the course of conjugation, the cell membranes and the underlying oral filamentous sheaths of both cells fuse, thus uniting the endopla sm of both cells in the mouth region. Synaptonemal complexes in the me iotic chromosomes are more distinct in H. vermiculare than in most oth er ciliates. The micrographs presented here depict clearly the central filament, transverse elements, and other substructures.