Fig. 4. Time-lapse series of meiosis I in wild-type and dd4 spermatocytes. (A-E) Wild-type division. (A) Prophase with opposed asters (asterisks). (B) Prometaphase with elongated nuclear region within which bivalents (arrows) acquire biorientation. (C) Rapid movement of homologues (arrows) towards opposite poles as the phase-dense material (pdm) starts accumulating along the spindle. (D) Anaphase B where the parafusorial membranes are fully visible. (E) Telophase in which the cleavage furrow (arrowheads) will separate the daughter nuclei (nu). (F-J) Late spindle collapse in a dd4^S spermatocyte. (F) Asters initially segregate. (G) pdm accumulates along the equatorial region of bipolar spindles as apparently bioriented homologues move between the poles (arrows). (H) The spindle poles approach each other before any visible segregation of homologues takes place. The pdm accumulates at the centre of the spindle, which becomes the apical regions of two nascent conical structures. (I) These cones elongate further and their apexes become darker. (J) The cell becomes disorganized as individualization of nuclear-like (nu) vesicles takes place. (K-L) Defective MTOC segregation or early spindle collapse in a dd4^S spermatocyte. (K) In this cell, a bona fide biastral spindle never forms at the onset of meiosis and the hemi-spindle structure seen in this panel persists until later stages. (L) Individualized bivalents tend to undergo rapid movements towards the pole containing the aster (arrow) as the pdm accumulate distally. (M) In later stages all visible chromosomes localize in the vicinity of the astral pole without evident segregation of the homologues (the curved arrow indicates the direction of a rotation of the cell). (N) The `hemi-spindle' then becomes conical as the pdm accumulates at the apex. (O) The pdm tends to fray and disorganize and several nuclear-like vesicles (nu) form in the region previously occupied by the asters. Bar, 10 µm.