Abstract
The author summarizes recent data gathered by direct measurement of O2 content, and by photographing the movement of radiopaque substances, in the blood of embryos delivered by caesarian section. (A method for studying the fetal circulation in utero and at normal birth remains to be discovered.) The discussion includes the pattern of the fetal circulation, anatomical and dynamic factors in its maintenance and variations, changes in the pattern at birth by caesarian section, timing and mechanism of the functional closure of the ductus arteriosus, foramen ovale, and ductus venosus.