Depending upon the size and shape of the mass, you may be seeing what is called a viviparous phenomenon in your papaya, that is to say, germinated or pre-germinated seeds within the papaya that are forming fruit or related developmental structures. They usually appear as an unexpectedly large, white, rounded nugget inside the papaya when you cut it in half. There is some thought that this is most likely to occur in areas of high humidity.

The papaya plant is dioecious, which means that some trees are male, some are female, and in the papaya’s case, some others are hermaphroditic. Only the females and the hermaphrodites can bear fruit.  The reproductive process in many plants is subject to unusual events.

Unlike the male papaya flowers, the female and hermaphroditic papaya flowers are not particularly showy, but they have an ovule that can grow into a large fruit filled with seeds that are capable of creating new plants. 

In India, there has been some observation of increased vivipary and seed disorders in papaya crops, but the increased occurrence has not yet been scientifically explained.

Courtesy of NYBG Plant Information Service