German biologist who developed a view that life was continuous and immortal. His theory of heredity was based on the separation of somatic and germ cells. He viewed the death of an individual organism as insignificant, since life consisted of the "germ plasm" forming the eggs and sperm. To refute Lamarck's theory of inheritance of acquired characteristics, he cut the tails off 22 generations of mice (1,592 in all) and showed that all young had normal tails. He suggested that chromosomes contain hereditary material, suggesting that the quantity of germ plasm was divided into two in the egg and sperm, which then recombined. His theories gave birth to Entwicklungsmechanik (developmental mechanics), which studied how heredity controlled development of the embryo.