Mechanisms for positioning the cell nucleus are important in ciliates because they insure that the genes are distributed to both daughter cells at division. The objective of this project is to analyze the mechanisms by which the macronucleus of the ciliate Stentor is positioned within the cell. These apparently involve: 1) attachment of the nucleus to a specific, visibly differentiated region of the cell surface (narrow cortical striping) and 2) control over the direction of macronuclear elongation during cell division, probably by the cell surface. The main technique used will be microsurgery with electron microscopy as a subsidiary technique. The following approaches will be used: 1) Using microsurgery, nuclei will be added to or removed from singlet or doublet cells, severed or displaced within the cell. Analysis of events involved in return of operated cells to normal should provide insights into the mechanisms of nuclear positioning. 2) Available evidence suggests that the macronucleus is attached to the cell surface in the region of narrow cortical striping. Electron microscopy will be used to investigate the physical basis for this attachment and to determine the mechanism by which nuclei transplanted into the endoplasm become attached to the cortex. 3) There is evidence suggesting that the cortex also acts to position the nucleus by determining the direction of macronuclear elongation during cell division. Elongation in turn apears to depend on the orientation of microtubules associated with the elongating macronucleus. Microsurgery in combination with electron microscopy will be used to test this hypothesis.