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Theories about fetal respiration began in antiquity. Aristotle characterized pneuma as warm air, but also as the enabler of vital functions and instrument of the soul. In Galen's system of physiology, the vital spirit was carried by the umbilical arteries, the nutrients by the umbilical vein from the placenta to the fetus. In 1569 Aranzio postulated that the maternal and fetal vasculatures are distinct. From 1670 to 1690, a century before the discovery of oxygen, researchers understood that during respiration some form of exchange with the air must occur, naming the substance biolychnium, phlogiston, sal-nitro, or nitro-aerial particles. An analogy of placental and pulmonary gas exchange was described in 1674 by Mayow. In 1779, Lavoisier understood the discovery of oxygen, discarded the phlogiston theory, and based respiration physiology on gas exchange. With the invention of the spectroscope, it became possible to measure hemoglobin oxygenation, and in 1876 Zweifel proved fetal oxygen uptake. Major progress in understanding fetal gas exchange was achieved in the 20th century by the physiologists Barcroft in Cambridge and Dawes in Oxford.