Abstract
1. Oswaldocruzia filiformis has ten autosomes in its diploid comple ment. In addition males possess a sex (X) univalent while females have two such chromosomes. 2. Segregation of the unpaired X in the male is variable. It may divide at first anaphase or at the second but not at both. This lability in behaviour of the X together with its occasional pre-meiotic non-disjunction is taken to indicate that the XO system in this species is of recent origin. 3. Chiasmata are probably localised distally in origin in both sexes and so terminalise very rapidly. For this reason one rarely finds interstitial associations persisting to first metaphase. Moreover rod bivalents are much more frequent than rings. 4. The appearance of the centromere at first metaphase and the attenuation of this region at second metaphase suggest that the meiotic centromere in this species may divide before first anaphase. 5. The nuclei of the epithelium which invests the gonads are endopolyploid and originate by endo-mitosis.
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