Degenerations in a class of matrix varieties and prinjective modules

Let K be an algebraically closed field and let I be a finite partially ordered set. To any vector we associate the irreducible variety MatvI with an action of the algebraic group GvI (see Section 6). The main aim of this paper is to give a description of degenerations of orbits of this action in terms of homomorphisms and extensions of prinjective KI-modules.