Ontogenetic organization of the FMRFamide immunoreactivity in the nervus terminalis of the lungfish, Neoceratodus forsteri.

The development of the nervus terminalis system in the lungfish, Neoceratodus forsteri, was investigated by using FMRFamide as a marker. FMRFamide immunoreactivity appears first within the brain, in the dorsal hypothalamus at a stage around hatching. At a slightly later stage, immunoreactivity appears in the olfactory mucosa. These immunoreactive cells move outside the olfactory organ to form the ganglion of the nervus terminalis. Immunoreactive processes emerge from the ganglion of the nervus terminalis in two directions, one which joins the olfactory nerve to travel to the brain and the other which courses below the brain to enter at the level of the preoptic nucleus. Neither the ganglion of the nervus terminalis nor the two branches of the nervus terminalis form after surgical removal of the olfactory placode at a stage before the development of FMRFamide immunoreactivity external to the brain. Because this study has confirmed that the nervus terminalis in lungfish comprises both an anterior and a posterior branch, it forms the basis for discussion of homology between these branches and the nervus terminalis of other anamniote vertebrates.