German experimental embryologist. A mechanist, he emphasized the study of developmental mechanics (Entwicklungsmechanik) by means of experimental interference with the development of embryos. He was involved in a debate with the vitalist Driesch in the 1890's over the causes of development. This was based on the fact that Roux had split frog eggs at the two-cell stage and obtained half-embryos, whereas Driesch had split sea urchin eggs at the same age and obtained fully-formed adults. Not to be confused with Pasteur's scientific collaborator Emile Roux, who worked on infectious diseases.