What’s the “world”?
The global namespace* contains a variable named World.
When you enter a Morphic “world”, World is set to point to the current “world”
When you send the openInWorld message to a Morph, it gets the current World and adds itself.
* For the curious, the global namespace is a dictionary named Smalltalk. Do Smalltalk inspect in any Workspace to get a look at it.