  The matrix model for IIB Superstring proposed by Ishibashi, Kawai, Kitazawa
and Tsuchiya is investigated. Consideration of planar and non-planar diagrams
suggests that the large N perturbative expansion is consistent with the double
scaling limit proposed by the above authors. We write down a Wilson loop that
can be interpreted as a fundamental string vertex operator. The one point
tadpole in the presence of a D-string has the right form and this can be viewed
as a matrix model derivation of the boundary conditions that define a D-string.
We also argue that if world sheet coordinates $\sigma$ and $\tau$ are
introduced for the fundamental string, then the conjugate variable
${d}/{d\sigma}$ and ${d}/{d\tau}$ can be interpreted as the D-string world
sheet coordinates. In this way the $SL(2Z)$ duality group of the IIB
superstring becomes identified with the symplectic group acting on ($p,q$).
