Abstract
The distribution of the chromosome number and the karyotype in 18 spontaneous mammary tumors developed at the inguinal regions of the male soft-furred rat (Millardia meltada) are analysed. Among them 17 tumors show a modal occurrence of 50 chromosomes which are the normal chromosome number of this species. The karyotype of the tumor cells is not different from the normal somatic cells. One specimen has 51 chromosomes among which one trisomy (no. 16) is included. The normal karyotype of the tumor cells does not change during 10 passages in vitro cultivation. Based on the karyotype analysis the mammary tumor of the soft-furred rat is suggested to be a benign type depending on the male hormone. Table I. Chromosome number distribution of the 18 mammary tumors developed in the males of the soft-furred rat
