Bipolarly oriented bivalents in spermatocytes of the turbellarian Mesostoma ehrenbergii, displaying fast oscillatory movements in metaphase, were studied with the electron microscope. Kinetochores and chromosome fibers were reconstructed using serial sections cut perpendicular to the spindle axis. Only a small proportion of kinetochore microtubules (kMT) is continuous between the kinetochore and the centrosome. kMTs intermingle with non-kinetochore microtubules (non-kMTs), partly inclined with regard to the kMTs, thus forming a chromosome fibre MT lattice. This resembles the "microtubular fir-tree" structures (MTFT) described by Bajer and Mole-Bajer in Haemanthus endosperm mitosis. A minimal function of the MTFT may be the anchorage of kMTs in the polar region. Regarding the association of MTs with the chromosome, three types of attachment can be discriminated: (1) "normal" insertion of kMT plus ends in the kinetochore, (2) penetration of kinetochores and deep insertion in the chromatin, and (3) lateral attachment with kinetochore and chromatin. Lateral association of MTs seems to be mediated by filamentous crossbridges. The observations are discussed in connection with possible behaviour of kMTs during kinetochore movement.
