Final answer:
Thomas Hunt Morgan and his colleagues employed microscopy to observe chromosomes during meiosis in fruit flies, providing evidence for the Chromosomal Theory of Inheritance.
Step-by-step explanation:
Thomas Hunt Morgan and his colleagues used microscopy to demonstrate that hereditary information was carried on chromosomes. They conducted extensive genetic crosses with Drosophila melanogaster, the common fruit fly, and observed the chromosomes during various stages of cell division under the microscope. Morgan's meticulous observations correlated the behavior of chromosomes during meiosis with the inheritance of traits, providing significant experimental evidence for the Chromosomal Theory of Inheritance.