  With the aim of clarifying the eleven dimensional content of Matrix theory,
we examine the dependence of a theory in the infinite momentum frame (IMF) on
the (purely spatial) longitudinal compactification radius R. It is shown that
in a point particle theory the generic scattering amplitude becomes independent
of R in the IMF. Processes with zero longitudinal momentum transfer are found
to be exceptional. The same question is addressed in a theory with extended
objects. A one-loop type II string amplitude is shown to be R-independent in
the IMF, and to coincide with that of the uncompactified theory. No exceptional
processes exist in this case. The possible implications of these results for
M-theory are discussed. In particular, if amplitudes in M-theory are
independent of R in the IMF, Matrix theory can be rightfully expected (in the N
-> infty limit) to describe uncompactified M-theory.
