Immunohistochemical staining of two serial sections cut in the transverse plane from an area associated with the posterior mandible, close to the temporomandibular joint in an adult Monodelphis domestica (South American opossum). Heads were hemidissected then rapidly fixed in methacarnoy to preserve antigenicity prior to staining [Sciote and Rowlerson, 1998]. a Reactivity of anti-type IIM MyHC antibody for fibers in masseter, a jaw-closing muscle. c Reactivity of anti-type II (IIA, IIX and IIB) antibody for fibers in digastric, a jaw-opening muscle. b Craniofacial skeleton of Monodelphis, with line indicating the relative area stained in a and c.