>>Where do the fertile seeds come from?
Rarely, and under some circumstances, the Triploid cell can undergo spontanious Meiosis before fertilization, becoming a Hexaploid cell. (6-chromosome pairs). This then gets fertilized and, since it's an even number again, it can successfully pair off and produce fertile seeds.<<
Yeah, I don't think that's what happens to produce the occasional viable seed in a seedless watermelon. The would be pollinated by a diploid melon which would only contain 1 set of chromosomes, so that would result in an an odd number of chromosomes again (7). What most likely happens is that occasionally you get 1 set of chromosomes during egg cell meiosis and when pollinated by the diploid male parent (again 1 set of chromosomes as per usual in sexual reproduction) you now have a viable diploid seed.
