On Maximal Abelian Selfadjoint Subalgebras of Factors of Type II1

In this note, we show that if Nis a proper subfactor of a factor . of type II1 with finite Jones index, then there is a maximal abelian self.adjoint subalgebra (masa) Aof Nthat is not a masa in .. Popa showed that there is a proper subfactor .0 of the hyperfinite type II1 factor . such that each masa in .0 is also a masa in .. We shall give a detailed proof of Popa's result.