Submanifolds with prescribed mean curvature vector field

In a recent paper [2] K. Nomizu has shown that a natural analogue of an n-sphere in an arbitrary Riemannian manifold is an n-dimensional umbilical submanifold with non-zero parallel mean curvature vector, which he calls extrinsic sphere sometimes. This note is concerned with the question whether extrinsic spheres have a special topological or differentiable feature.