Abstract
Cytological aspects of gametogenesis are studied in the common oxyuroid (Nematoda) parasites of Mus musculus, Syphacia obvelata (Oxyuridae) and Aspiculuris tetraptera (Heteroxynematidae). Both species are shown to be haplodiploid: males develop from unfertilized eggs and are haploid and females develop from fertilized eggs and are diploid. In female A. tetraptera there are 12 chromosomes in oogonial divisions; meiosis is normal and six bivalents were observed in ova. There are two maturation divisions and embryos in eggs in utero contain 6 or 12 chromosomes. In males, there are six chromosomes in spermatogonial divisions; there is a single maturation division and all spermatozoa have six chromosomes. Gametogenesis was similar in Syphacia obvelata but the chromosome numbers are four in males and eight in females.

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