  Foliations on the space of $p$-divisible groups were studied by Oort in 2004.
In his theory, special leaves called central stream play an important role. In
this paper, we give a complete classification of the boundary components of the
central streams for an arbitrary Newton polygon in the unpolarized case.
Hopefully this classification would help us to know the boundaries of other
leaves and more detailed structure of the boundaries of central streams.
