German zoologist whose attempts to discover how organs and tissues are assigned their structural form and functions at the time of fertilization made him a founder of experimental embryology.

A student of German biologist Ernst Haeckel, Roux studied in Jena, Berlin, and Strasbourg. He was an assistant at the Institute of Hygiene in Leipzig (1879–86) and professor at the universities of Breslau (1886–89), Innsbruck, Austria (1889), and Halle (1895–1921).

Believing that mitotic cell division of the fertilized egg is the mechanism by which future parts of a developing organism are determined, Roux began in the 1880s an experimental program using frog eggs. He destroyed one of the two initial subdivisions (blastomeres) of a fertilized frog egg, obtaining half an embryo from the remaining blastomere. From his results, he concluded that determination of future parts and functions had already occurred in the two-cell stage and that each of the two blastomeres had already received the determinants necessary to form half the embryo. His theory was later negated, however, when German biologist Hans Driesch, working with sea urchin eggs, obtained small but fully developed embryos from both initial blastomeres.

In 1894 Roux founded Archiv für Entwicklungsmechanik der Organismen, the first journal of experimental embryology.