Reguier De Graaf, a Dutch physician, born at Schoonhoven in 1641, died in Delft, Aug. 17, 1673. He was especially distinguished for having originated the discovery that reproduction takes place in the viviparous as well as in the oviparous animals by means of ovarian eggs, and that all animals are therefore essentially oviparous. The "Graafian vesicles" of the mammalian ovary were discovered and described by him, although he mistook their exact nature and considered them as true eggs, while they have since been shown to be only the receptacles within which the microscopic egg is contained. He also acquired a wide reputation by his investigations on the pancreatic juice. His works are : Disputatio Medic a de Natura et Usu Succi Pancreatici (Leyden, 1664); De Virorum Organ is Generationi in-servientibus, etc. (1668); Epistola de nonnullis circa, Partes Genitales novis inventis (1668); Tractatus Anatomico-Medicus de Succi Pancreatici Natura et Usu (1671); and De Mulierum Organis Generationi insertientibus (1672).