Abstract. Tipulid spermatocytes form normally functioning bipolar spindles after one of the centrosomes is experimentally dislocated from the nucleus in late diakinesis (Dietz, R., 1959, Z. Naturforsch., 14b:749-752; Dietz, R., 1963, Zool. Anz. Suppl., 23:131-138; Dietz, R., 1966, Heredity, 19:161-166). The possibility that dissociated pericentriolar material (PCM) is nevertheless responsible for the formation of the spindle in these cells cannot be ruled out based on live observation. In studying serial sections of complete cells and of lysed cells, it was found that centrosomefree spindle poles in the crane fly show neither pericentriolar-like material nor aster microtubules, whereas the displaced centrosomes appear complete, i.e., consist of a centriole pair, aster microtubules, and PCM. Exposure to a lysis buffer containing tubulin resulted in an increase of centrosomal asters due to aster micotubule polymerization. Aster-free spindle poles did not show any reaction, also indicating the absence of PCM at these poles. The results favor the hypothesis of chromosome-induced spindle pole formation at the onset of prometaphase and the dispensability of PCM in Pales.

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SYNOPSIS. The intraflagellar structure (IFS) of the flagella of Trypanosoma brucei was examined on the basis of ultrathin sections in various planes. The IFS is composed of filaments approximately 50 A thick. These filaments seem to be identical with the protofilaments found earlier to be the basic elements of the contractile flagellar fibrils. The fibrillar system is firmly connected with the IFS and the latter is attached to the flagellar membrane by filaments. The lattice‐like appearance of the IFS is caused by longitudinal and oblique filaments running in different planes. The structure of this network is discussed in detail. The IFS may serve as an abutment for the contractile flagellar fibrils.

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The region between the kinetochores of syntelically oriented autosomes and the pole in meta- and anaphase of Pales ferruginea spermatocytes was studied by means of serial sections. Microtubule (MT) were counted and measured, and the spindle region was reconstructed by superimposition of successive micrographs. Kinetochoric (kMTs) and non-kinetochoric microtubules (nkMTs) interdigitate with one another forming a bundle which is often arrow-shaped due to an inclination of nkMTs (skew nkMTs) with respect to the kinetochore-pole axis. The average length of MT in the bundle decreases towards anaphase while the average number increases. The extent of MT disorder in anaphase half-spindles is higher than in metaphase. The number of kMTs inserted in the kinetochore was found to remain unchanged from meta- to early anaphase. Some of the kMTs become divergent in anaphase. The relative proportion of skew nkMTs within the kMT/nkMT bundle is higher in anaphase. It is proposed that the morphological changes observed to occur from meta- to anaphase are due to fragmentation of kMTs followed by disorientation of the MTs pieces. Some aspects of the physical properties of the half-spindles are discussed.