SPB insertion defect of brr6.ts8. brr6.ts8 cells that had been grown to mid-log phase at 25°C were synchronized with respect to cell cycle progression by centrifugal elutriation and shifted to 36°C. Cells were fixed by high-pressure freezing 110–150 min later and processed for electron microscopy. A–H and I–Q are consecutive, serial sections through two mitotic cells, respectively. In both cells, a nonfunctional SPB has detached from the nuclear envelope (B and C, and J and K). Although the active SPB is anchored to the nuclear envelope on just one side in the upper cell (right side in D and E), it is not possible to discern an association of the active SPB with the envelope in the lower cell in O–Q. In both cells, the active SPB is associated with a hole in the envelope (to its left in D and E and around it in L–Q). The following features are indicated: The dense brackets in B and J indicate the separation between the nonfunctional SPB and the nuclear envelope. The thin brackets underneath the nuclear envelope in C and I indicate a region occupied by osmophilic material below the SPB on the outer, cytoplasmic face of the nuclear envelope. The pale linear structure curving through the panels indicated by the arrow is the nuclear envelope (NE). The thick black arrows in B–D and L–O indicate the edges of the hole in the nuclear envelope through which the nucleoplasm (N) spills into the more granular cytoplasm (C). The arrows in B, C, M–O, and Q indicate the boundary of the nucleoplasm extruded into the cytoplasm. NP, nuclear pore; MT, microtubules. Bars, 200 nm.