  In this paper, for any pair $(\zeta, \xi)$ of Newton polygons with $\zeta
\prec \xi$, we construct a concrete specialization from the minimal
$p$-divisible group of $\xi$ to the minimal $p$-divisible group of $\zeta$ by a
beautiful induction. This in particular gives the affirmative answer to the
unpolarized analogue of a question by Oort on the boundaries of central
streams, and gives another proof of the dimension formula of the central leaves
in the unpolarized case.
