and chemical explanations for organismal development. At the time, there was considerable scientific debate over the question of how an embryo developed. Following Wilhelm Roux's mosaic theory of development, some believed that hereditary material was divided among embryonic cells, which were predestined to form particular parts of a mature organism. Driesch and others thought that development was due to epigenetic factors, where interactions between the protoplasm and the nucleus of the egg and the environment could affect development. Morgan was in the latter camp; his work with Driesch demonstrated that blastomeres isolated from sea urchin and ctenophore eggs could develop into complete larvae, contrary to the predictions (and experimental evidence) of Roux's supporters. A related debate involved the role of epigenetic and environmental factors in development; on this front Morgan showed that sea urchin eggs could be induced to divide without fertilization by adding magnesium chloride. Loeb continued this work and became well-known for creating fatherless frogs using the method. When Morgan returned to Bryn Mawr in 1895, he was promoted to full professor. Morgan's main lines of experimental work involved regeneration and larval development; in each case, his goal was to distinguish internal and external causes to shed light on the Roux-Driesch debate. He wrote his first book, The Development of the Frog's Egg (1897). He began a series of studies on different organisms' ability to regenerate. He looked at grafting and regeneration in tadpoles, fish, and earthworms; in 1901 he published his research as Regeneration. Beginning in 1900, Morgan started working on the problem of sex determination, which he had previously dismissed when Nettie Stevens discovered the impact of the Y chromosome on sex. He also continued to study the evolutionary problems that had been the focus of his earliest work. Columbia University Morgan worked at Columbia University for 24 years, from 1904 until 1928 when he left for a position at the California Institute of Technology. In 1904, his friend, Jofi Joseph died of tuberculosis, and he felt he ought to mourn her, though E. B. Wilson—still blazing the path for his younger friend—invited Morgan to join him at Columbia University. This move freed him to focus fully on experimental work. When Morgan took the professorship in experimental zoology, he became increasingly focused on the mechanisms of heredity and evolution. He published Evolution and Adaptation (1903); like many biologists at the time, he saw evidence for biological