The law of three stages is an idea developed by Auguste Comte in his work The Course in Positive Philosophy. It states that society as a whole, and each particular science, develops through three mentally conceived stages: (1) the theological stage, (2) the metaphysical stage, and (3) the positive stage.
The ductus arteriousus is a blood vessel that connects the pulmonary artery to the descending aorta. Otherwise known as the ductus Botalli, this structure pumps blood into the fetus' non-functioning lungs from the right ventricle. This blood vessel will close at birth and becomes the ligamentum arteriousum.
