Oh, Jose, I don't think you're right about marrow and gelatin being the same stuff. The marrow in bones is where blood cells are manufactured. Gelatin comes from the colligen, which is the soft stuff hiding between all the the calcium in the bones, and can also be found in muscle tissue. There's colligen in the marrow, but lots of other stuff there, too. (When making a pot roast, the temperature should be low in order to release the colligen in the meat - high temps makes the stuff sieze up and toughen, not desirable.)
Pigs knuckles and feet are loaded with colligen, and is where we used to get the gelatin used in desserts like mouses and Jell-O.