De humano foetu.

According to Charles Singer, Aranzi gave the first adequate printed account of the gravid uterus, and finally dispelled the idea of a human cotyledonous placenta. He gave by far the best description of fetal anatomy up to that time, especially examining the fetal heart, where he saw the ductus arteriosus and foramen ovale (and described their occlusion after birth). Aranzi believed the maternal and fetal circulations to be separate. He also described the ductus venosus of the fetus, and the corpora Arantii in the heart valves. Incidentally, he was the first to record a pelvic deformity. Digital facsimile of the Leiden, 1564 edition from Google Books at this link.