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September 22, 1962


JAMA. 1962;181(12):1071. doi:10.1001/jama.1962.03050380049015

"By the end of sixth week of fetal life, when the embryo is barely a centimeter in length and the heart no bigger than a match head, it has all the external and most of the internal gross appearances of the adult heart."1

Why should the right ventricle and pulmonary artery be differentiated 8 months before a pulmonary circulation is required? Are there clinical implications in this early differentiation into a fourchambered organ? Grant has answered these questions through perceptive and highly original embryologic research. In a recent report this investigator described the physiologic adjustments to the heart's uniquely early development. He stressed that every mammal has 2 different double-circulations, one during fetal life and another during extrauterine life. Structural abnormalities in the heart are not as dangerous for the fetus as for the newborn, since in the former the 2 paths of flow of the double cardiac circulation

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