American biologist; born in Germany April 7, 1859; educated at the universities of Berlin, Munich, and Strasburg (M. D. 1884). He took a postgraduate course at the universities of Strasburg and Berlin, and in 1886 became assistant at the physiological institute of the University of Würzburg, remaining there till 1888, when he went in a similar capacity to Strasburg. During his vacations he pursued biological researches, at Kiel in 1888, and at Naples in 1889 and 1890. In 1892 he was called to the University of Chicago as assistant professor of physiology and experimental biology, becoming associate professor in 1895, and professor of physiology in 1899. In 1902 he was called to fill a similar chair at the University of California.
The main subjects of his works are: animal tropisms and their relation to the instincts of animals; heteromorphosis, i.e., substitution at will of one organ of an animal for another; toxic and antitoxic effects of ions; artificial parthenogenesis; and hybridization of the eggs of sea-urchins by the sperm of starfish.
Among Loeb's works may be mentioned: "Heliotropismus der Thiere und Seine Identität mit dem Heliotropismus der Pflanzen," Würzburg, 1889; "Physiologische Morphologie," part i., ib. 1890; part ii., ib. 1891; "Vergleichende Physiologie des Gehirns und Vergleichende Psychologie," Leipsic, 1899; edition in English, New York, 1900.