A PasteUpMorph can be pasted on other PasteUpMorphs.
A reference to the topmost instance of PasteUpMorph is kept in the global variable World. This instance of a PasteUpMorph then has a WorldState and MorphWorldView . See also Project.
Details: Things gets further complicated by the fact that you can have worlds in worlds and, starting with 2.9a, multiple active worlds.
(Actually the whole class is a mess and as Stephane already noticed, it contains a lot of stuff which should be refactored into specialized classes).
Looking at the instance variables, we see that a world can have a presenter and a model (I don't understand that) and much other stuff like a background or some not yet working (or again broken) things like support for hypercard-like stacks. More important are the variables of the WorldState.