An illustrating exampleWe illustrate our considerations in the following special situation: Let E := C n×m be the space of all complex n × m-matrices where 1 ≤ n ≤ m are fixed integers in the following. Every matrix z is considered as an operator C m → C n between finite-dimensional Hilbert spaces and z is the corresponding operator norm, i.e.is the open unit ball of E where z * = z is the transposed conjugate of z. It is known that the ball B is homogeneous under biholomorphic automorphisms and hence is an example of a bounded symmetric domain. Consider on E the linear transformation groupThen Γ is a complex Lie group that has precisely n + 1 orbits E 0 , . . . , E n in E -each E r is the (locally closed) complex submanifold of all matrices of rank r. Clearly, E r−1 is in the closure of E r for every 1 ≤ r ≤ n and E n is the unique open orbit. For every pair of integers p, q ≥ 0 let G p,q be the Grassmannian of all p-planes in C p+q . Then G p,q is a compact symmetric hermitian manifold of dimension pq holomorphically equivalent to G q,p . Every E r is in a canonical way a Γ -equivariant holomorphic fibre bundle Supported by a grant from the German-Israeli Foundation (GIF), I-0415-023.06/95.