The placenta is an organ of remarkable architectural arrangement in that, as Slemons1 has expressed it, the structure accommodates simultaneously the fetal and the maternal circulations, yet holds them apart. Since the days of the Hunters it has been known that the mother's blood never enters the fetus and also that the reverse phenomenon is impossible. As pregnancy is essentially a problem in nutrition under conditions in which the dominant metabolic forces are those favoring growth, the question as to how the nutrient substances are conveyed to the fetus has aroused much speculation. How do they pass the apparent placental barrier? In the main, two antagonistic theories have been developed; one of these, the vitalistic, assumes that the wall of the chorionic villus takes an active part in the placental interchange; the other, the mechanistic, regards this wall as a passive, semipermeable membrane conforming with the laws of osmosis
THE NUTRITION OF THE FETUS. JAMA. 1929;92(22):1865. doi:10.1001/jama.1929.02700480055014
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