Original sentence: He studied at Berlin and Munich universities, receiving his doctorate in 1918.He was the first to use both identical and unidentical twins in a single study (to determine the role of genes over environment in skin moles).His influence on the invention of the twin study is not widely acknowledged, possibly due to his support during the Second World War of the Nazi party's eugenics program.However, in 1942 he had lost his chair at Leiden and was temporarily held hostage due to his resistance to the German occupation policies, and after the war he regained his position, which he held until 1962.In 1929 he became chair of dermatology at Leiden university in the Netherlands.Siemens published "Zwillingspathologie" (Twin Pathology) in 1924 which introduced the widely used "classical twin method"
Edited sentence: By 1942 he had lost his chair at Leiden and was even temporarily held hostage due to his resistance to the German occupation policies.He was the first to not only use twins in a study (to determine the role of genes over environment in skin moles), but to distinguish identical from fraternal twins, and use the correct 100%/50% genetic similarity inferences regarding these groups.His influence on the invention of the twin study is not widely acknowledged, possibly due his being in German, with the associations that the Second World War and the Nazi party's eugenics program convey.In 1929 he became chair of dermatology at Leiden in the Netherlands.In a break-through study, he published "Zwillingspathologie" (Twin Pathology) in 1924 which introduced the widely used "classical twin method".Not until after the war did he regain his faculty position at Leiden, which he held until retirement.Siemens studied at  Berlin and  Munich universities, receiving his doctorate in 1918