  We prove the existence of Morita equivalences between the spin blocks at the
extremal points of strings in the block-reduced crystal graph. When the
parities of the core partitions are not preserved, these equivalences require
crossovers, with a block of the symmtric group Morita equivalent to a block of
the alternating group and vice versa. The result permits in improvement of the
bound for Donovan's Conjecture given by Kessar.
