John Mayow was the first to realize that "nitro-aerial" vapour, or oxygen, is essential to respiration of a living animal, and he was soon led to inquire "how it happens that the foetus can live though imprisoned in the straits of the womb and completely destitute of air."[33] As a consequence of this interest, the third of his _Tractatus Quinque medico-physici_, published in 1674, is devoted to the respiration of the fetus _in utero_. He shows truly remarkable insight when he concludes therein that It is very probable that the spermatic portions of the uterus and its carunculae are naturally suited for separating aerial particles from arterial blood. These observations premised, we maintain that the blood of the embryo, conveyed by the umbilical arteries to the placenta or uterine carunculae transports to the foetus not only nutritious juice, but also a portion of the nitro-aerial particles: so that the blood of the infant seems to be impregnated with nitro-aerial particles by its circulation through the umbilical vessels in the same manner as in the pulmonary vessels. Therefore, I think that the placenta should no longer be called a uterine liver, but rather a uterine lung.[34] Although Mayow's attempted analysis of respiration of the chick embryo _in ovo_ is less than successful, his views on fetal respiration were soon accepted by many, and his tract stands as a great contribution to physiological embryology. The studies of such individuals as John Standard reporting the weight of various parts of the hen's egg, e.g., the shell, the yolk, the white, reveal the wing of embryological investigation that was increasingly obsessed with quantification and the physicochemical analysis of the embryo and its vital functions. In this they were following the injunction of Boyle, who used the developing embryo as a vehicle in an attack upon the idea that mixed bodies are compounded of three principles, the obscurities of which operated to discourage quantification: