Editing Thomas Hunt Morgan (section)

===Bryn Mawr===
In 1890, Morgan was appointed associate professor (and head of the biology department) at Johns Hopkins' sister school [[Bryn Mawr College]], replacing his colleague [[Edmund Beecher Wilson]].<ref>{{Cite journal | last1 = Morgan | first1 = T. H. | author-link = Thomas Hunt Morgan| title = Edmund Beecher Wilson. 1856–1939 | doi = 10.1098/rsbm.1940.0012 | journal = [[Obituary Notices of Fellows of the Royal Society]] | volume = 3 | issue = 8 | pages = 123–126 | year = 1940 | title-link = Edmund Beecher Wilson | s2cid = 161395714 | doi-access = free }}</ref> Morgan taught all morphology-related courses, while the other member of the department, [[Jacques Loeb]], taught the physiological courses. Although Loeb stayed for only one year, it was the beginning of their lifelong friendship.<ref>Allen, ''Thomas Hunt Morgan'', pp. 50–53</ref>  Morgan lectured in biology five days a week, giving two lectures a day. He frequently included his recent research in his lectures. Although an enthusiastic teacher, he was most interested in research in the laboratory. During the first few years at Bryn Mawr, he produced descriptive studies of [[sea acorn]]s, ascidian worms, and frogs.{{cn|date=May 2024}}

In 1894 Morgan was granted a year's absence to conduct research in the laboratories of ''[[Stazione Zoologica]]'' in [[Naples]], where Wilson had worked two years earlier. There he worked with German biologist [[Hans Driesch]], whose research in the experimental study of development piqued Morgan's interest. Among other projects that year, Morgan completed an experimental study of [[ctenophore]] embryology. In Naples and through Loeb, he became familiar with the ''Entwicklungsmechanik'' (roughly, "developmental mechanics") school of experimental biology. It was a reaction to the vitalistic ''[[Naturphilosophie]]'', which was extremely influential in 19th-century morphology. Morgan changed his work from traditional, largely descriptive morphology to experimental embryology that sought physical and chemical explanations for organismal development.<ref>Allen, ''Thomas Hunt Morgan'', pp. 55–59, 72–80</ref>

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.<ref>Allen, ''Thomas Hunt Morgan'', pp. 55–59, 80–82</ref>  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.<ref>{{cite journal
|author=Loeb, Jacques|author-link=Jacques Loeb
|journal=American Journal of Physiology | volume=31 | pages=135–138 | year=1899
|title=On the Nature of the Process of Fertilization and the Artificial Production of Normal Larvae (Plutei) from the Unfertilized Eggs of the Sea Urchin|issue=3
|url=http://www.stanford.edu/group/Urchin/loeb.htm|doi=10.1152/ajplegacy.1899.3.3.135
|hdl=2027/hvd.32044107304297
|hdl-access=free}}</ref>
<ref>{{cite book|title=Artificial parthenogenesis and fertilization|author=Loeb, Jacques |publisher=University of Chicago Press | year=1913 | url=https://archive.org/details/artificialparth00loebgoog|quote=jacques loeb sea urchin.}}</ref>

When Morgan returned to Bryn Mawr in 1895, he was promoted to full professor. Morgan's main lines of experimental work involved [[regeneration (biology)|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 system|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.<ref>Allen, ''Thomas Hunt Morgan'', pp. 84–96</ref>