Exceptional Meiotic Conditions in Oocytes of Ambystoma talpoideum, Triturus viridescens Xenopus laevis and Their Relation to the Origin of Spontaneous Heteroploidy
Ten exceptional secondary oocytes from three genera of the Amphibia (Ambystoma talpoideum, Triturus viridescens and Xenopus laevis) are described. Six oocytes contained a single meiotic spindle in metaphase II, with the diploid number of dyads. Since no polar bodies were observed on these eggs, their condition is probably the result of an aberration of the first meiotic division which resulted in failure of formation of the first polar body. Four oocytes contained two spindles. One of these probably originated as a binucleate oogonium, since two polar bodies were present, but the remaining three are probably the result of an abnormal first meiotic division, in which polar body formation did not occur. These observations give cytological support to previously suggested routes by which heteroploidy may originate.