Where do the circular patterns around the seeds in the watermelon flesh come from?

Wil asks: why do watermelon seeds have those pale circular rings around them in the red flesh? The answer takes us from a tiny flower on a vine to the secret of how the whole watermelon is built — those circles are the original cradles where each seed was born and fed. Featuring a sunny Afro-pop highlife song: Ring Ring Ring! Little Halo Ring!