sentences:  Isenkrahe visited in 1856 the Progymnasium in Jülich, in 1857 the Marzellengymnasium in Cologne and from 1858 to 1863 the Realprogymnasium in Bonn.In 1868 he studied at the University of Bonn where he chose the subjects mathematics, physics, chemistry, mineralogy, botany, zoology, philosophy, Latin and German.On 31 July 1866 he made his PhD with an award-winning work about the anatomy of Helicina titanica, a species of snail.