Fig. 4. As shown by TEM, the mitotic spindle is semi-open and basal bodies with axonemes are present at the four spindle poles. (A) Cross-section of a mitotic nucleus; microtubules form a sheath surrounding the nucleus. The nuclear envelope is intact except for large openings at the poles. Bar, 200 nm. (B) High-magnification view of area outlined by box in A. Cytoplasmic microtubules enter the nucleus through a large opening in the nuclear membrane. Bar, 50 nm. (B') Diagram of nuclear membrane (blue) and microtubules (red) seen in B. (C) Spindle microtubules radiate from the basal body seen in cross-section, which organizes the spindle microtubules. Bar, 300 nm. (D) Basal bodies are located between the two daughter telophase nuclei. One basal body with associated axoneme is seen in longitudinal section, the others are in cross-section. Bar, 400 nm. bb, basal body; fl, flagellum; n, nucleus.