  In this paper, we consider a special class of Capelli bitableaux, namely the
Capelli-Deruyts bitableaux. The main results we prove are the hook coefficient
lemma and the expansion theorem. Capelli-Deruyts bitableaux of rectangular
shape are of particular interest since they are central elements in the
enveloping algebra. The expansion theorem implies that these central element is
explicitely described as a polynomial in the classical Capelli central
elements. The hook coefficient lemma implies that the Capelli-Deruyts
bitableaux are (canonically) expressed as the products of column determinants.
