Tomographic slices of the spindles studied here. (A) 10-nm slice from a serial tomogram of a C. reinhardtii metaphase (CrM1, see Fig. S1 for model). Chromosomes (CH), the nuclear envelope (NE), and an opening in the envelope (PO) through which MTs project toward the spindle pole are evident. (A′) 4-nm slice through a different metaphase spindle (CrM2 in Fig. S1). One MT ends near a chromosome (a KMT) but others continue. The arrow indicates a flared PF. (B) 10-nm slice from a serial tomogram of a one-cell embryo of C. elegans. Chromosomes (CH) are evident, and a centriole (arrow) marks the spindle pole. (B′) 4-nm slice through the chromosome–MT interface of a different C. elegans metaphase (CeM in Fig. S2). Chromatin, at left, is separated by a fibrous zone from the KMTs with flaring plus ends. No outer plate is seen. (C) 10-nm slice from a tomogram of a metaphase budding yeast (ScM1, Fig. S3); SPB, spindle pole body; NE, nuclear envelope. (C′ and C″) Two 4-nm slices from a prometaphase budding yeast (ScPM1); slender filaments connect the capped MT minus ends with the spindle pole. (D) 10-nm slice from a prometaphase fission yeast (SpPM in Fig. S4 C); SPB and the NE are evident, but chromosomes are not. (D′) A 4-nm slice from the same cell. Some MTs end in pairs where the texture of nucleoplasm is different (CH). Such MTs have been interpreted as KMTs. Bars: (A) 0.5 µm; (A′) 200 nm; (B) 1 µm; (B′) 100 nm; (C) 0.5 µm; (C′ and C″) 100 nm; (D) 0.5 µm; (D′) 100 nm.