germ-plasm theory

Concept of the physical basis of heredity expressed by the biologist August Weismann (1834–1914). It claimed that germ plasm, which Weismann believed to be independent from all other cells of the body, was the essential element of germ cells (eggs and sperm) and was the hereditary material passed from generation to generation. First proposed in 1883, his view contradicted Lamarck's then-prevalent theory of acquired characteristics. Though its details have been altered, its idea of the stability of hereditary material is the basis of the modern understanding of physical inheritance.

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Theory that certain diseases are caused by invasion of the body by microorganisms. Louis Pasteur, Joseph Lister, and Robert Koch are given much of the credit for its acceptance in the later 19th century. Pasteur showed that organisms in the air cause fermentation and spoil food; Lister was first to use an antiseptic to exclude germs in the air to prevent infection; and Koch first linked a specific organism with a disease (anthrax). The full implications of germ theory for medical practice were not immediately apparent after it was proven; surgeons operated without masks or head coverings as late as the 1890s.

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