Zoologist, born in Frieburg, Germany. He studied at Greifswald and Berlin, and taught zoology at Bonn from 1859. His best-known work is on unicellular organisms. In 1861 he argued that cells in general contain a nucleus and protoplasm as 'the basis of life' and that a boundary membrane is not always present. His duplicity theory of vision of 1866, based on his study of the retina of birds, ascribed separate functions to the retinal rods and cones, and was a step towards later theories of vision.