B. TEM analysis of SPBs. Left, SPBs in SPO13 and spo13∆ cells lacking all APC/C activators. TEM samples were collected at 12 h in SPM. The spo13∆ mutation causes MP formation at metaphase I (P < 0.0001, Fisher's exact test). Right, SPBs at metaphase II. CDC20-mAR and CDC20-mAR mpc54∆ mpc70∆ cells were synchronized by arrest/release at metaphase I, and TEM samples were collected at metaphase II (80 min after release). MP formation is blocked in the absence of Mpc54 and Mpc70 (P < 0.0001, Fisher's exact test). Scale bar, 0.2 μm. 