Definition: the basic plan of the circulation of a young mammalian embryo, at first similar to that in aquatic forms, with an unpartitioned heart and conspicuous aortic arches in the branchial or pharyngeal region; as gestation progresses, the arrangement of the major blood vessels gradually approaches that of an adult, but the routing of blood through the heart, characteristic of an adult, cannot be attained until lung breathing begins at birth.
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