Regnier de Graaf. Engraving of Regnier de Graaf (1641-73), Dutch anatomist. Graaf studied at the University of Leiden and later obtained a degree from the University of Angers, France. Graaf studied the pancreas and gall bladder, and was the first to collect the secretions that these organs discharge into the intestine. In 1668, he was the first to describe the structure of the testicles and in 1673 described the ovary (an organ which he named). The minute follicles in the ovary are known as Graafian follicles. Graaf also introduced the work of Leeuwenhoek to the Royal Society. Graaf died of the plague when aged only thirty- two.