  We study M theory on $AdS_4 \times \RP^7$ corresponding to 3 dimensional
${\cal N}=8$ superconformal field theory which is the strong coupling limit of
3 dimensional super Yang-Mills theory. For SU(N) theory, a wrapped M5 brane on
$\RP^5$ can be interpreted as baryon vertex. For $SO(N)/Sp(2N)$ theory, by
using the property of (co-)homology of $\RP^7$, we classify various wrapping
branes and consider domain walls and the baryon vertex.
