"Grégoire... was primarily concerned to illustrate by reference to the ungula that volumetric integration could be reduced, through the ductus in planum, to a consideration of geometric relations between the lines of plane figures. ...Unfortunately, the delayed publication of the Opus geometricum prevented it from receiving... attention... In 1647, ten years after the publication of Descartes' La Géométrie, algebraic methods were rapidly gaining ground and the form and manner of presentation of Grégoire's work was not such as to make easy reading. ...Amongst those who gained much from the Opus geometricum... [was] Blaise Pascal whose Traité des trilignes rectangles et de leurs onglets is based essentially on the ungula of Grégoire. Huygens recommended the section on geometric series to Leibniz who later came to make a thorough study of the entire work. Tschirnhaus... found in the ductus in planum a valuable foundation for the development of his own algebraic integration methods."