An opening between the right and left atria that's present in fetal mammals is the

It is the Foramen Ovale. The foramen ovale is a little gap situated in the septum between the two upper councils of the heart. The foramen ovale is utilized amid fetal dissemination to accelerate the go of blood through the heart. 
The one fatal cardiac shunts, the other being the ductus arteriosus. Another comparative adjustment in the embryo is the ductus venous. In many people, the foramen ovale closes during childbirth.

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