Ovid's Metamorphoses: does it have continuity or not?

[nuda pedem]

Well, I'm making a little investigation about Ovid's Metamorphoses. In the first four verses I see that he states a problem about his own poem: Is it a carmen perpetuum or a carmen deductum in Callimachus'  way? I've read some papers about this, but authors don't agree... I think there's something that puts some unity between the stories, but it's difficult to know exactly what it is. How did Ovid do it? Is the idea of metamorphosis enough? Or is it the chronological disposition of the epyllia? What do you think?

If you can't understand what I say, please let me know, I'm not a native English speaker.