  The issue of justifying the matrix-theory proposal is revisited. We first
discuss how the matrix-string theory is derived directly starting from the
eleven dimensional supermembrane wrapped around a circle of radius
$R=g_s\ell_s$, without invoking any stringy assumptions, such as S- and
T-dualities. This derivation provides us a basis for studying both string
($R\to 0)$- and M ($R\to \infty$)-theory limits of quantum membrane theory in a
single unified framework. In particular, we show that two different boosts of
supermembrane, namely one of unwrapped membrane along the M-theory circle and
the other of membrane wrapped about a transervse direction which is orthogonal
to the M-theory circle, give the same matrix theory in the 11 dimensional
limit, $R\to \infty$ (with $N\to \infty$). We also discuss briefly the nature
of possible covariantized matrix (string) theories.
