Thomas Hunt Morgan ( 25 September 1866 â `` 4 December 1945 ) was an American geneticist and embryologist . Thomas Hunt Morgan ( September 25 , 1866 -- December 4 , 1945 ) was an American evolutionary biologist , geneticist and embryologist and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries relating the role the chromosome plays in heredity .