Mitotic apparatus formation and cleavage induction by micromanipulation of the nucleus and centrosome: the centrosome forms a spindle together with only the chromosomes at a short distance. We micromanipulated the nucleus and centrosomes in the zygote of the starfish, Asterina pectinifera, in order to investigate their roles in mitotic apparatus formation and cleavage induction. The zygote cleaved without spindle formation when its nucleus was removed. When one or two centrosomes were transplanted, they formed asters in the recipient cell, which cleaved into three or four blastomeres so that each blastomere might contain one centrosome or aster. When one centrosome was removed, a half-spindle formed in the manipulated cell, which did not cleave until the other centrosome was duplicated. When both centrosomes were removed, no microtubular structures such as the spindle and the aster appeared in the manipulated cell, which failed to cleave. These results indicate that two centrosomes or more in the cell induce cleavage with or without the nucleus and that one centrosome or less does not induce cleavage. It is also concluded that the centrosome(s) together with the nucleus forms a half-spindle or bipolar spindle. However, from the experiments of nucleus transplantation and displacement, spindle formation is found to depend on the distance between chromosomes and centrosomes. The half-spindle formed when the distance from the centrosome to the chromosomes was shorter than 22 microns; on the other hand, when the distance was longer than 22 microns, the nucleus remained apart from the aster, which means that the functional range of the astral microtubule''s ability to engage chromosomes was 22 microns from the centrosome.