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.
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