August Weismann (1834-1914), German zoologist. Weismann spent a few years practising medicine before entering research. He became a teacher at the University of Freiburg, and was forced to be a theorist by failing eyesight. He worked in heredity, proposing that all organisms contain a 'germ plasm'. He later identified this with the chromosomes of the cell nucleus. He proposed the chromosome number halved by division in reproduction, with one half from each parent merging. He saw this as the basis of variability. Although broadly correct, Weismann incorrectly thought chromosomes were immune to outside influence.