. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. SALLY HUGHES-SCHRADER b. Re-approach of homologous kinetochores; metaphase plate formation. The re-contraction of the bivalents, re-approach of the homologous kineto- chores, and the gradual movement toward the equatorial region proceed precisely as in Stagmomantis. ///. Bivalent structure and chiasmata In spite of the markedly early separation of the kinetochores and the conse- quent opening out of the bivalents before spindle formation, the chromatid structure of the bivalents is not analyzable at this The separati
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. SALLY HUGHES-SCHRADER b. Re-approach of homologous kinetochores; metaphase plate formation. The re-contraction of the bivalents, re-approach of the homologous kineto- chores, and the gradual movement toward the equatorial region proceed precisely as in Stagmomantis. ///. Bivalent structure and chiasmata In spite of the markedly early separation of the kinetochores and the conse- quent opening out of the bivalents before spindle formation, the chromatid structure of the bivalents is not analyzable at this The separation of homologous chromosomes, at first localized in the kinetochore region, continues until a large kinetochore loop is formed, or, in some bivalents, the homologues become completely disjoined except for one short paired segment. Sister chromatids remain closely associated and cannot be traced through the per- sistently paired regions. That the separation of homologous chromosomes is not solely due to kinetochore movement is shown by the presence in some bi- valents of a second locus of opening out in addition to the kinetochore loop or half loop. This is apparent in the separation of the ends of the horizontally placed bivalent at the mid left in Figure 24, and in all bivalents of Figure 25. After spindle formation and during the stretching process the open cross formation. i 23 456789 10 (IX FIGURE 27. Liturgousa annulipes. Eleven bivalents and X at completed metaphase. Gentian violet. is encountered in several bivalents (Figures 26 and 28, a and b). The frequency of open crosses is per cent (in 249 counted) during the stretch, and is reduced to per cent (in 351) by final metaphase. The open cross configuration is usually accepted as evidence of a chiasma in process of resolution by the rotation of the arms of the bivalent. We might then assume, as White (1941) has suggested, that the terminal connections between the homologous chromosomes of the bivalents at metaph
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