  We study properties of a map from a certain unitary group in $n$ variables to
a related unitary group in $\binom{n}{k}$ variables. We explain how it gives
rise to a map between canonical models of Shimura varieties and we prove that
it extends to the ordinary locus of the integral model. Finally we extend
results of Satake on the endomorphism ring of a generic image point to positive
characteristics.
