Freud entered the University of Vienna at age 17. He had planned to study law, but joined the medical faculty at the university, where his studies included philosophy under Franz Brentano, physiology under Ernst Brücke, and zoology under Darwinist professor Carl Claus. In 1876, Freud spent four weeks at Claus's zoological research station in Trieste, dissecting hundreds of eels in an inconclusive search for their male reproductive organs. In 1877 Freud moved to Ernst Brücke's physiology laboratory where he spent six years comparing the brains of humans and other vertebrates with those of frogs and invertebrates such as crayfish and lampreys. His research work on the biology of nervous tissue proved seminal for the subsequent discovery of the neuron in the 1890s. Freud's research work was interrupted in 1879 by the obligation to undertake a year's compulsory military service. The lengthy downtimes enabled him to complete a commission to translate four essays from John Stuart Mill's collected works. He graduated with an MD in March 1881.