The nuclear envelope is composed of sheet-like ER that separates chromatin from tubulin during interphase. During oocyte maturation in C. elegans , a fenestrated envelope of sheet-like ER continues to envelope the meiotic spindle through metaphase I. ER is thus excluded from the nuclear/spindle volume during spindle assembly. To test the importance of this exclusion, we forced ER into the meiotic spindle by coupling kinesin motor domains to the ER. Forcing ER into the spindle interior caused a statistically significant increase in metaphase spindle width. Exclusion of ER from the spindle thus affects spindle geometry.