EMBRYONIC CIRCULATION. “Embryonic remnants of mammalian circulation. some of the best examples of non-functional developmental ‘vestiges’ involve mammalian circulatory changes. These are detours or shunts (pulmonary to systemic, or ‘right to left’) occurring in the shift from fetal to neonatal mammalian circulation.”
https://evolution-outreach.springeropen.com/articles/10.1186/s12052-014-0012-5