The present paper deals with the karyotype analysis of 22 Japanese raccoon dogs collected in Ishikawa, Yamaguchi, Osaka and Wakayama prefectures, Japan. The basic chromosome number of these specimens is 2n=38, but the higher number of the chromosomes than the basic one has been observed in all specimens, because the B-chromosomes are included in all of them. Among the 22 specimens, the Robertsonian fission of one metacentric element has been observed in 3 specimens.
The external ora of the uterus in the rat, Rattus norvegicus, are examined morphologically. They are polymorphic characteristics due to the uterus didelphys with two external ora (Type A), the uterus bipartitus with one external os (Type B) and the intermediate form between them (Type C). The WM/Ms and WKA/Ms strain rats are the Type A, the WKS/Ms strain rats are the Type C, but the other 6 strains, such as ALB/Ms, BUF/Ms, F-344/Ms, NIG-III, ACT/Ms and LEJ/Ms are the Type C. By mating between the Type A and Type B and also Type C and Type A, it is proved that the Type A is a completely dominant character to the Types B and C.