The Organ of JACOBSON of the guinea pig was examined mesoscopically, light- and electronmicroscopically. A computer aided three dimensional reconstruction was performed. The organ lies hidden ventromedially in the nasal cavity covered by the mucous membrane of the nasal septum, as in mammals with a secondary palate, and as in all terrestrial vertebrates, it is closely related to the os incisivum or primary palate, respectively, and the septomaxillary bone. Being a sensory organ for long distance perception lying in the chondrocranium, it possesses its own capsule deriving from the original fundus of the nasal cavity. It contains two different mucous membranes. These are the sensory mucous membrane, Tunica mucosa sensoria vomeronasalis, with a primary sensory epithelium composed of sensory cells, glia cells and basal cells and moreover of the multipolar, bi- and pseudounipolar ganglion cells of the terminal nerve and a glandular mucous membrane, Tunica mucosa nonsensoria vomeronasalis (glandularis). The latter corresponds by its arrangement and structure to the mucosa of the respiratory tract of the nasal cavity and is characterized especially by its Lamina fibroelastica, its Lamina muscularis mucosae and the Plexus venosus tumidus. It guarantees as auxiliary structure the proper function of the olfactory sense organ, which is corroborated by the structure and innervation of the sensory mucous membrane. This is significant especially when regarding the minor importance - also reflected by the required space it takes in the nasal cavity - of this sensory organ in mammals compared to other terrestrialvertebrates. Both parts of the mucous membrane of the Organ of JACOBSON have their own independent blood supply and innervation. The sensory mucous membrane is supplied by the vomeronasal artery and vein, A. et V. vomeronasalis, originating in the external ethmoidal artery and vein and in addition to the sensory vomeronasal nerve the terminal nerve, respectively. The glandular mucous membran is supplied by the nasopalatinal artery and vein, A. et V. nasopalatina, originating from the maxillary artery and vein and the nasopalatinal nerve, N. nasopalatinus, branching off the maxillary nerve (V2). It is concluded that the autonomous Organ of JACOBSON of the terrestrial vertebrates is partially homologous to the olfactory sense organ in fish, which is the phylogenetically primary olfactory sense organ as opposed to the newly-acquired secondary olfactory sense organ of the terrestrial vertebrates, the Regio olfactoria. The findings in this thesis are summerized in a suggested alteration of valid nomenclature.