I can only speak for the institutions I've studied/worked at, but while I'm pretty sure our donated cadavers were subject to the usual requirements (whole, no autopsy, weight ramge, etc), the majority of them most certainly did not remain in one piece. Our anatomy labs had different sections where we kept preserved, pre-dissected specimens for undergrads in different programs to get hands on demonstrations, though the actual dissections were only done by more advanced students. There were some whole pre-dissected cadavers with the Y incision used in autopsies for demonstration of visceral organs, some heads where you could remove the skull cap and see the brain, limbs with everything removed but ligaments, bones and muscles so you could see what happens as you articulate the joint, individual organs or bones, whole arms or legs, etc. Even when preserved and kept moist these specimens don't stay pliant enough to work with forever so retired pieces were periodically mass cremated and interred in a crypt belonging to our organization in a local cemetery. We didn't reunite all the parts of one body before cremation because the wet specimens only last months whereas plastinated ones might hang around the lab for many years, so it would be impractical if not impossible, but I have no idea if other places do things differently. I never personally got to see any fat ass corpses though, at least until I happened to see the Body Worlds exhibit, which is too bad because I think it's a valuable learning experience.