Full Idea
Worm-theoretic Perdurantism says spatio-temporal continuants are mereological fusions of instantaneous temporal parts or stages located at different times; Stage-theoretic Perdurantism says they are instantaneous temporal stages of continuants.
Gist of Idea
Worm Perdurantism has a fusion of all the parts; Stage Perdurantism has one part at a time
Source
Thomas M. Crisp (Presentism [2003], 2.1)
Book Reference
'The Oxford Handbook of Metaphysics', ed/tr. Loux,M /Zimmerman,D [OUP 2005], p.216
A Reaction
[Armstrong, Lewis and Quine defend the first; Sider the second] The Stage view seems to be the common sense view. Sider suggests that the earlier stages are counterparts, not the thing as it currently is.