The skeleton of a variety of groups

If £ is a class of groups then sD and q£ denote the classes of all groups isomorphic to, respectively, subgroups and factor groups of groups in £ • If G is a group, an element of QS{G} will be called a eeotion of G . A class I) is called eeation closed if QSD = I) and, as is well-known, for any class I) , QSI) is section closed. For basic facts and terminology relating to varieties of groups we refer to [9] where, however, a section is called a factor. For any variety IJ we write F(lJ) for the class of all finite groups in H , M(lj) for the class of all finite monolithic groups in IJ and C(tJ) for the class of all critical