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Of the two umbilical veins formed in young rat embryos, the light regresses early while the left persists until birth. In a number of rat fetuses from mothers subjected to a transitory deficiency of pteroylglutamic ("folio") acid during pregnancy, this pattern was reversed and the right, instead of the left, umbilical vein persisted until birth. In such cases, the ductus venosus was always absent. No record of persistent right umbilical vein has been found in the literature.doi:10.1161/01.res.5.2.187 pmid:13414094 fatcat:xahwuhmrc5gtpgdjouklblbaem