During the first half of the 20th century, after the rediscovery of Mendel’s laws in 1900, there was a growing interest in determining how these laws of genetics applied, among other things, to mammalian inheritance. In 1909 Morgan’s decisive genetic work on the fruit fly, Drosophila melanogaster, with which his name would always be associated, began. Thus, from the beginning of the 20th century, even before the establishment of fundamental knowledge of cell and tissue biology, daring experimentalists sought to explore the nature of tumor tissue by performing intra- and interspecific tumor grafts.