The Collaborative International Dictionary of English v.0.48:

Mesenteron \Mes*en"te*ron\, n. [NL. See Meso-, and Enteron.]
   (Anat.)
   All that part of the alimentary canal which is developed from
   the primitive enteron and is lined with hypoblast. It is
   distinguished from the stomodaeum, a part at the anterior end
   of the canal, including the cavity of the mouth, and the
   proctodaeum, a part at the posterior end, which are formed by
   invagination and are lined with epiblast.
   [1913 Webster]