[Cytogenetic studies of brown field-mouse]

The karyotype of Brown Field-Mouse (Microtus mandarinus) is reported in this paper. The formula of karyotypes is:2n = 50 = 2 (M,T)+2SM+44T+XX(M,SM)XY(SM,ST). The first pair of the autosomes and the X-chromosomes show polymorphism. Only one X-chromosome can be found in the seven out of the fifteen female specimens studied. The formula of its karyotypes is 2n = 49 = 2(M,ST)+2SM+44T+X O. The X-chromosomes of these seven specimens are metacentric, and are different from the X (that is SM) of normal male, these are the type M. Three kinds of karyotypes of Microtus mandarinus studied in this paper are different from the karyotypes (XO,OY,XY) of M. oregoni studied by Brown et al. (1964). The mechanism of generation of chromosomal polymorphism is discussed. Individual ontogenesis mechanism of the type XO and its reproduction are also discussed.