Here are some facts about Navel Oranges I found:
Navel oranges are characterized by the growth of a second fruit at the apex, which protrudes slightly and resembles a human navel. They are primarily used for eating, as the skin is thicker and easier to peel than a common orange.
According to Dorsett, Shamel, and Popenoe (1917) of the U.S. Department of Agriculture, a single mutation in 1810 to 1820 in a Selecta orange tree planted at a monastery near Bahia in Brazil, probably yielded the navel orange, also known as the Washington, Riverside, or Bahia navel. The mutation causes the orange to develop a second orange at the base of the original fruit, opposite the stem, as a conjoined twins in a set of smaller segments embedded within the peel of the larger orange. From the outside, it looks similar to the human navel, hence its name
The navel oranges of today have exactly the same genetic makeup as the original tree, and are therefore clones, all navel oranges can be considered to be the fruit of that single nearly two-hundred-year-old tree. ( Wikipedia)