Realizing that the internal structure of nucleons undergoes appreciable changes through embedding into nuclear matter the finite nucleus is considered from the outset as a correlated system of quarks. As an appropriate approximation scheme we introduce an extended bag model, i.e. a bag with nuclear radius containing all 3n quarks of the n-nucleon system. As the direct interaction we choose a SU (3)-invariant generalized pairing force which is supposed to contain the major effects of the more general term. As a result of the calculations containing a.o. a rigorous diagonalization of the total Hamiltonian, the characteristic properties of the conventional (nuclear) shell structure are reproduced: The spin orbit splitting appears now as a property of the relativistic single quark states, and the independent nucleon states are represented by correlated 3-quark-clusters. In addition, the nuclear radius law follows directly from a normal value of the vacuum pressure.