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.

First edition of Aranzi's Anatomicarum observationum published with the third edition of De humano foetu. In the Anatomicarum observationum Aranzi pointed out that the eye muscles arise from the margin of the optic cavity, not from the dura mater as was thought previously; and he described the extensor indicis proprius, obturator externus, genioglossus, coracobrachialis, and tensor fascia latae. Most importantly, he provided the first description of the hippocampus in the inferior horn of the lateral ventricle, which he referred to as the hippocampal ventricle, and the inferior extension of the lateral ventricular choroid plexus. He called the elevation in the floor of the inferior horn the “sea horse/hippocampus” or “white silkworm/bombycinus” and observed that it extends rostrally as the fornix. Overall, his description of the ventricular system was a clear improvement on that of Vesalius, who had also described the inferior horn. (Clarke and O’Malley 719-21; Larry W. Swanson). Digital facsimile from Google Books at this link.