A peculiar type of the cell division in the rhizopod, Borkovia desaedeleeri (Sarcodina, Gymnophryidae)

A peculiar type of cell division-the closed pleuromitosis with an extranucl ear spindle-was observed in the small freshwater reticulopodiate rhizopod, Borkovia desaedeleeri. The bipolar microtubular spindle with electron dense centrospheres on its poles originates in the cytoplasm adjacent to the nuc leus. No morphological connection between centrospheres and kinetochores or dictiosomes is observed. Chromosomes are placed in the nucleoplasm, and th eir kinetochores are fixed in the nuclear envelope in the course of mitosis . Chromosomal microtubules have a contact with the sites of chromosomal fix ation in the nuclear envelope and provide for correct dispersion of chromos omes to poles of the nucleus. Then the nucleus is divided by a medial const riction. The formation of chain-like aggregations, characteristic of B. des aedeleeri, is shown to be a result of the incompleted cell division. The si milar mode of the cell division in Parabasalians is discussed.